Part of being a GM is saying 'no'. Players like to push the limit to see what they can get. It's just the way it is. So, get used to it because saying 'no' is gong to come up again and again and again.
Very much this. As a GM, it is far, far, far better to simply tell your player "I want you to build a character with a similar power level to the rest of the group." than it is to ban specific combinations they attempt.
Just tell your player your real objection (I think your character is too powerful for this campaign) and set him free to build a character in-line with the rest of the party. There are plenty of complicated but low-power builds your player may enjoy, but he may never attempt one of them if he doesn't know that's what you're looking for.