One of the hazards of letting someone have the script if you have purchased it.
What was the agreement when you purchased it to install it? Did several people put money in together and to purchase it? There are several questions that would have to be answered that has a bearing on what you can do.
If you were the ONLY person that put the money forward you should have some recourse.
Do you not have access to that server? If not, notify the hosting company that he no longer has the right to use that script and it's now being pirated since you are no longer authorizing his use of it. They should either take the site offline or tell him to remove the script and to go to something else to use. Of course, if he has backups he can simply go to another hosting provider and set it back up, but then he's in a piracy setting and that can be reported to the XF staff and they can take care of it.