You have two options: 1. Lie to them to pass UAT, or 2. Tell them their expectations are unrealistic and every service they currently use has the same issue, whether they realize it or not. If they aren't going to use what you have made because it can't guarantee deletion, then they should be consistent and opt out of absolutely everything else as well. It is impossible to prove the non-existence of anything, and that is just as true in this context as it is in philosophy.