Stake can trace the activity into your account and IP address within their platform. And I think, they're not going to impose anything that you haven't done.
But if they've seen you broke their rules, they're going to surely impose a ban to you if the offense is too big that would lead in banning. I also feel bad for the remaining amount that you can't withdraw but try to ask them nicely maybe there's some mercy they can show to you.
They should at least try to give some explanation about what caused the ban though, like providing evidence of wrongdoing so the offender can see whether they've done something wrong and if not then they can at the least dispute the ban. I think a mercy or at least a consideration with warning would be the most useful in this situation, not saying that Stake did something wrong but I think that it's fair that we somehow do this to everyone if they don't have a serious infraction.
When there is an issue like this, reputable casino like Stake will always prove what they accuse to the players.
Most of the time, players will complain and ask about the ban then the casino will provide the reason through email or live support.
In this case, OP said that he is accused to do bonus fraud but he did not mention it with more detail.
I'm sure he get the information from Stake with more details and I doubt Stake ban account without valid reason and evidences.