Since Watch2Gether does not require users to log in when joining a room, it is difficult to enforce a permanent ban on a particular individual.
Although you can ban specific user accounts, there’s nothing to stop someone just joining in a new tab as an anonymous (temporary) guest.
However, you can enable “Members Only” in the moderation settings of the room, which allows you to approve every new user entering the room. This way you can at least have some control over who is let into the room in the first place.