I had a player that did exactly the same thing a few months ago and while I can't guarantee it'll work, I'll describe what I did and hopefully it will.
The player in question was constantly setting things up very carefully so that it was nearly impossible to figure out that they were breaking something until after they had already done it, and I didn't exactly want to retcon it. What I ended up doing was taking them aside and "rewarding" them for it. The main reason players like to make god-PCs is that they want to be special, and instead of trying to constantly deny them of that, the easiest solution is to let them have it, but in a way that is constructive.
For instance, the player I was talking about from my campaign we ended up incorporating into the plot by making them into a mole for the villain, so the other players inevitably ended up fighting and killing her just before they did the same to her boss. If a PC doesn't work as a PC because they're far too weak or far too strong compared to the others, my first solution is to make them a plot device instead.
Another solution that works well that I've seen is to adjust the power scales accordingly. Above all, don't nerf the player who needs the nerf, because they'll just get pissed off and either loudly complain or just munchkin their way to the top again. Instead, buff all the other players by setting things up so they "just happen" to get better equipment or more opportunities to gain XP that the broken player does not.
Lastly, if that doesn't work, just sit down and talk about putting in some house rules to restore the balance. Remember, balance for the sake of balance is boring, so there's no shame in making the development of house rules a dynamic process to keep the players on their toes.
Above all, throughout any of these solutions, take care to not make the overpowered player feel targeted. Fundamentally, they aren't; these are just measures to make the game more fun for everyone rather than to pick on someone.