They can blacklist, because they can effectively force users to not use anonymity. I have stated that several times. I feel like you are arguing disingenuously. I refuted your point about them needing to do it before coins have already been mixed, because:

1. There are always new coins from mining.

2. They could incentivize users to escape the blacklist gridlock for the mixed coins, if the user antes up their password. This can cascade into de-anonymization of others, etc...