B., the ideas recommended by other moms are great, like charts and books like "1,2,3" (haven't used it myself but hear good things about it). You have some positive disciplines to choose from there. I wanted to add one thing that applies no matter what method you choose....
You've used a lot of different things -- talking to him like an adult; promising him candy if he behaves well; threatening smacks if he behaves badly; time outs, etc.
But have you done any one thing consistently? Do you stick with any one discipline -- and try to think of it as disciplines to teach him to BE disciplined, rather than punishment because you're angry -- for any length of time? Or do you change around the discipline or reward or your reaction over and over because nothing seems to work? Nothing WILL work unless it's consistent and repeated and he knows exactly what to expect each and every time he behaves a certain way. Right now it sounds like his behaviors get different reactions at different times because you're trying to find the one reaction that works on him. He needs to know the rules and if the rules (meaning what happens when he misbehaves) change all the time, he will keep on pushing his boundaries to see what you will do next.
Know that he will keep pushing at first once you settle on a course of consistent discipline; you will think, this isn't working either. But you have to give it time to sink in with him that his choices will have these specific consequences. And you have to find consequences he'll care about. As you said, he doesn't care about losing the candy bar. He might care more about losing his favorite toy, or TV time, for a week, etc., but you need very clearly to connect the loss with his specific actions.
Attention is attention. I learned the hard way with my own daughter that we adults tend to think kids should not want "negative attention" like time outs, parental displeasure, taking away favorite toys, etc. But for many kids, negative attention is still attention and they crave attention in any form. So to our surprise, the kids misbehave and get disciplined but then keep misbehaving. If we're yelling at them or struggling with them about time out, we're paying attention to them. So discipline that gives him an unpleasant consequence AND takes your attention completely away is good (for instance, in time out, you do not talk to, scold or even react to him until you, not he, decides the time out is over).
Be sure he's getting lots of positive attention when you "catch him doing something good" and as someone else noted, get him a LOT of physical exercise. He may be a kid who needs a lot of physical activity to burn off the talk-talk-talk energy.
One other thought. The constant talking may mean he feels he loses attention to his siblings, so be sure he gets "alone time" with you and especially his dad--tough with the work schedule but it's a big need for little boys. And he may be very bright and imaginative--do you encourage him to pretend or make up stories or otherwise work out his talkativeness in a way that's not always talking at you? And does he have play dates with other kids his age to build social skills like letting others talk and take the lead?
Finally, please don't hope that school will fix this. Classes are too big and teachers too busy to work out individual kids' discipline problems in any detail. If you find a positive discipline and apply it consistently, you can do this.