How do I mechanically handle player's improvisation?

[Tanarii]

Speaking of which, one tactic I use is to remind players of stuff they might not have considered but their PC would probably know, like that any wizard who's as powerful as this guy supposedly is will likely have very robust defenses for protecting his stuff, and they're unlikely to have any success stealing it.

Absolutely, warn the players before you TPK them.

Also worth asking if they've paid dues with the local thieves guild and gotten authorization for this job? Since the powerful Wizard will have either paid them off, or have good enough defenses it doesn't need to. Worth knowing if you're gonna have thieves and assassins crawling out of the woodwork if you survive.

Always a good idea to point out things when players haven't even considered the basics.

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Either that or they're already so powerful they can do whatever they want, and really the only thing stopping them from murderhoboing around town is a moral code, armies, or constantly adventuring in the outer planes. Kinda joking, but really there's a point in D&D where that line gets crossed. Usually around Name Level.