It does not entirely beat the purpose of a salt, because still the existence of a rainbow table for that particular combination of algorithm and salt is a lot l

It does not entirely beat the purpose of a salt, because still the existence of a rainbow table for that particular combination of algorithm and salt is a lot less likely than for the algorithm unsalted. As for your question, a solution could be to allow the account at first and perform regular checks in the background. It won't make a big difference if the account is never created or deleted within 10 minutes. In fact, the latter could be better, because it will be harder to check for any outsider (including the offender) whether an address is blocked, which is good for privacy and helps keep the offender busy, because they can't use a different address right way—they first spend some time in vain.