Have you considered starting your own pool to point client rentals at?
Anyway, for users that wish to use anything at ghash.io, it's really your judgment call as to whether they are a risk. If they are, don't allow them. Otherwise, when people select them as the pool, you could have them check a checkbox that they understand that directing power there is increasing their mining power share, yada yada.
Starting our own pool is something we did not consider. From what I understand CEX.io require their users to use GHash.io.
I do like the point your made about informing the customer and a checkbox. Thanks for the suggestion.
As you point out the customer purchased the contract with intentions of mining to "Their" pool.
Thank you for the feedback and suggestion.
We appreciate your input.
-James