What do you mean "unfairly"? Much as we don't like it, the books are allowed to determine who they want and don't want as customers, so long as they treat us fairly while we are active there (abide by their rules, prompt payouts, etc).
SIA blatantly tells people that they're no longer wanted because they've won too much. Amount used to be 10-15K, maybe it's higher now. But I believe they still do it.