I presume that you are talking about mortgages for first time buyers not ISA's!
I'm sure that the Mortgage Broker / Lender will make the necessary credit checks on the applicants, including whether they have had any previous mortgages. Naturally this would not cover a previous property bought for cash, but then would they need a mortgage for their next purchase?
In answer to your question, the rules probably can't be broken, and there would probably be a clause in the small print allowing the loan to be called in if the terms of the loan agreement were found to be contravened.