Have a strong EULA that removes any type of liability from you for providing the software, and then let them do what they do on their servers, what do you care?
What you propose would be like some forum script telling me what I could or could not talk about on my site, wtf?!
If they are hosted with you tho, yeah, run a tight ship
Anyhow, In the future I would love a way to just setup my own server and deal with my own clients and remove the third party risk of an exchange.
How that is going to happen, who knows...