The same way as the porn bans: require the AI-therapy service provider to verify the user's location and block them if they're in a certain state

what if they dont comply and simply ignore, as a business incorporated somewhere in Ireland or Wisconsin ?

what is a mechanism to block incompliant website?

< what is a mechanism to block incompliant website?

They don't need to block it. If the company is accepting money from Americans and it's not using crypto, then there are business entities that have a presence in the US and are subject to US law enforcement. It may just be payment processors, but it's easy for the US govt to cut off access to most of its citizens as consumers.