Clark Kent's fighting style depends on his powers to allow him to deliver non-lethal force in situations that often require anonymity. In early seasons, the anonymity was especially important, as Clark had not developed a dual identity that would allow him to openly use his powers.
Clark believes strongly in non-lethal combat, and has never directly killed anyone using his powers. As a result, he has adopted a fighting style that depends largely on knocking opponents unconscious using his super strength; Clark will frequently grab his opponents and throw them through into a wall or object, rendering them unconscious. He also uses a head slap to achieve the same effect.
Clark will, for the most part, use super speed (the momentum usually knocking the person out cold) in order to remain anonymous or to sneak up on his opponents. He has also used his heat vision when fighting an opponent from long distances. Generally, Clark's confrontations with various opponents depend primarily on what powers they possess and the position he confronts them in; when facing opponents who possess strength similar to or greater than his, he tends to rely on his speed to gain an advantage, while heat vision is his 'default' skill when taking long-range action. For example, Clark gave a super-punch to Deadshot that sent him flying into the air, then crashing into a gate.
Clark uses much more force when battling more powerful metahumans or aliens, as seen when he apparently killed Titan in a brutal fight (although it should be noted that Titan was easily able to endure Clark's blows without actually being too badly injured by the attacks, and even his death was only the result of Titan falling on his own bone 'claws' rather than anything Clark did).
Clark has also been known to resort to other means when facing particularly challenging foes, such as when he attempted to use kryptonite against Zor-El while he himself had been temporarily rendered powerless by Jor-El's victory ring and his use of blue kryptonite against Bizarro, as well as his use of the Crystal of El against Zoners.
Now it should be noted that, in Salvation, when Major Zod threw Lois into the phone booth, Clark came in and landed a powerful uppercut that sent Zod flying across the city. In Finale Part 2, when he learned how to fly during a battle with Darkseid, he gathered force and speed, and rammed straight into Darkseid's vessel, destroying Darkseid's host body completely (although the fact that this action would not kill Darkseid and the vessel himself was already dead may account for Clark's use of such a brutal method of attack).
For the first eight years of his heroic career, Clark is largely dependent on his powers in his fighting style, as he has no formal combat training, in contrast to Lex Luthor and Lois Lane, who are both highly trained in martial arts. When Clark is stripped of his powers, he largely avoids physical combat and confrontation; in Spell, he confronted Countess Margaret Isobel Thoreaux and her cohorts in the Kawatche Caves with a shotgun; however, since they were possessing his friends, it is highly unlikely he actually intended to use it on them.
However, on occasion, Clark has fought without powers and won, most notably in Mortal when he managed to defeat three metahumans despite being temporarily rendered powerless; on these occasions, he generally relies on surprise to wait for the opportunity to strike, primarily exploiting the fact that nobody expects him to fight back.
Also, even without his powers he has developed a very powerful "right hook", his phyisical condition, when powerless, is similar, if not superior, to that of a highly strong human man, mostly due to his already muscular shape and peak-developed body. He has knocked people out with one blow in several episodes, like "Mortal", two guards at the Black Creek facility in season eight's "Odyssey", and delt with many opponents in the Phantom zone. This is also emphasized when he went to beat up Lex, at the Luthor Mansion, a single punch from Clark was able to put Lex on his nees, while the same kind of blow from Lex, didn't even move Clark from his standing point, although he did got a bleeding cut-lip.
After completely emerging himself in his Kryptonian training with Jor-El, Clark learned hand-to-hand combat, which allowed him to go one-on-one with any opponent who would have easily outclassed him in the past. This was later further emphasized when he was engaging Alia and they were both without powers due to exposure of blue kryptonite, and also he was evenly matched in a battle with Zod; in the past, while confronting the original General Zod, Clark was easily defeated. Later, it is shown that Clark is putting his training to use: He super-punched Deadshot with no apparent restraint on his face. Later, when Darkseid is about to possess Clark, he takes a fighting stance, furthering his ability as a skilled hand-to-hand combatant. While Clark was unable to defeat the Female Furies trained by Granny Goodness, this was mostly due to the green kryptonite in the area, which means he doesn't even have the strength to even stand as a human since kryptonite weakens him to the point of being weaker than a human. (Abandoned)
It is now assumed that Clark's fighting ability is equal to, or even greater than, that of Zod. While powerless in the Phantom Zone, Clark shows he is able to hold his own and even defeat Zod's gladiators; he also shows he is able to match and almost defeat Oliver Queen, who is noted to be a master hand-to-hand combatant. It should also be noted that Oliver landing a deathblow on Clark was the plan all along, so it is possible that this insinuates Clark could indeed defeat Oliver in hand-to-hand combat. (Dominion)
In the comics
While most authors prefer to portray Superman as having mediocre combat prowess in favor of showing the magnitude of his powers in battle, there have been some exceptions. Superman has many times been depicted as being a very capable fighter with or without his abilities. Examples include his learning boxing from Wildcat, his learning the fighting arts from Mongul, his learning additional martial arts from Wonder Woman, as well as his learning pressure point fighting from Batman. He has also learned two Kryptonian martial arts as well: Torquasm Vo and Torquasm Rao, enabling him additional fighting capabilities for both his body and mind.
- Showrunner Al Gough: "The fact is we adopted a policy from the start that the villain had to fall on their own sword, or die by circumstances beyond Clark's control. I've heard lots of arguments in early episodes that Clark did "kill" someone, but the fact is he hasn't and he won't." source: http://www.tvguide.com/news/smallville-questions-answered/071018-01