I'm with @Misadventure to approach a theme breaking player OOC first, and apply consequences, be they OOC or IC, afterwards. I like to think the faux pas is unintentional unless proven otherwise.
The hobby is not always kind to new players unaware of the finer ins and outs. WoD shows rough disdain for someone a bit muddy on the rules, and it can't hurt for helpful admin to pull aside Merry and let her know changelings tend to be sterile. Harry needs to know throwing fireballs in the street at the meter maid is not kosher for the local Consilium and yeah, we need a paradox roll for that.
If a game is sandbox, how much theme enforcement is really needed?
Non-sandbox games need enforcement at different levels. Staff who don't approve every silly concept under the sun are the first gatekeepers. Have no fear to say 'You can't be a Japanese swordsman in our 16th century lords and ladies game, even if historically plausible.' Theme matters for a reason. Characters made to fit it suffer when unthematic concepts and RP become the rule of the day.
I can't tell you the number of times I've seen someone throw a fit because the Masquerade / rule of shade / lex magica got enforced. As long as it's clear we all play by a given set of rules and assumptions, hold those ICly accountable. All those IC power positions like sheriff are there to help enforce. NPCs in power positions are ways to keep theme intact. Or use RL laws and common sense consequences for shooting up Main Street.