By these standards MEXC can accuse any user of anything at any time and take all their money without providing proof if they wanted to.
Yes they can, they are centralized exchanges and they can do anything and you can't object, that's what centralized exchanges are. They can block you for no reason or for the most trivial reasons.
There are many stories like this about people whose accounts were frozen for unknown reasons under the pretext of suspicious activity or even for political reasons, as happened with some Russian citizens, or as happened recently with the Palestinians, whose accounts were frozen by Binance for no reason.