As with drugs, those who have used a mixer for a small amount of money or who have displayed a signature here should not be worried (*). But, again, if the justice wants to be zealous, Bitcointalk as an entity can be considered as part of the network and the forum can be closed. This is the kind of thing, I think, Theymos wants to protect against.
(*) Not a legal advice
This is scary when i hear that the government officials can come up with an order to close the forum just because people are advertising mixers here. But then will they ban social media, facebook, twitter also if they see mixers being advertised their too?
If they really need to do the crackdown, the US government needs to ban the mixers and not the entities which are advertising it. By the way, they can only ban the mixers operating in or from the US. They can't ban the mixers worldwide just like they are unable to ban the cryptocurrencies.