The same thing that women do when they’re not interested.

Say something to the effect of “thanks, but I’m not interested” or “I’m flattered by your interest, but I’m hanging out with my friends tonight.” You don’t have to make a big deal out of him being a man, although if it’s a “mixed” gay/straight bar, letting him know that you’re not into to men might be appropriate if said in a matter-of-fact way. Then go back to what you’re doing and don’t act intimidated. So far it’s worked for me.

If he doesn’t take no for an answer, then let the management know you’re being bothered. That’s one reason I tip well, it tends to make the bartenders be protective of their good customers. If they make a scene with him, then it is wise to be safety conscious leaving afterwards and perhaps have someone walk you to your car.