how do banks do it now? you know the answer, at a high level. it's the same way we prevent unauthorized access when the blockchain isn't involved, and it's how we prevented unauthorized access before the blockchain existed.

it's how banks prevent someone from withdrawing money from your account without your permission, everywhere across the entire globe at once, all the time.

it's the same reason that I, a normal user, can't delete people from active directory at work, or change their passwords to mess with them.

you've dealt with this kind of permission system your entire computer-using life. you know how it works. you know what it is. being distributed doesn't matter at all.