He shouldn't have to do a thing. Europe can block X if they choose. On the other hand - if he wants the "everything app" to succeed in Europe as much as in the rest of the world - he'll need to completely de-link the companies that run the banking/etc part from the company that runs the social media part, and make sure only the non-social media parts have any presence in the EU. That won't stop their harassment but it will make it more difficult.

Currently what X has been doing is deleting content on request from the EU, but clearly that's not good enough...