I know how you feel.
So, re. banning stuff (which you don't want to do). The first question to ask yourself is this: playing the character like that is obviously what he enjoys. Is it really lessening the enjoyment of others? If so, then sure: address it. But if not, then the next question is: so what benefit does reducing his enjoyment have?
Your other suggested approach is to modify encounters in some way. Nerfing him with tons of undead is another "make the game unfun for him" solution. So what you want is a way that he can run around feeling awesome, without dominating encounters so that the other players feel inconsequential. Note that this goes back to the first point: it's only an issue if the other players feel that way. If it's not lessening anybody else's fun, let him have his fun.
So how does he get to be awesome without diminishing the other PC contributions? Splitting the party up is a PITA. I would regularly include a couple of extra bad guys who are pretty much there to be sneak attacked to death. Their function is to make him feel awesome. Stick 'em on a ledge or something so he can rogue his way up there, hiding and climbing. He'll love it and think you're an awesome DM, and your other players will be able to have the fight unimpeded.
Problem with that approach is you have to come up with something different each time. That's tough. You could try slipping in PC focused encounters - so occasionally one is largely solved by him sneak attacking, but the next has undead. So you're not just nerfing him, you're also facilitating him.
All that aside, the very first thing you should do is talk to him and explain your difficulty. He might understand and voluntarily change his character. Asking nicely should always be approach #1!