[gte910h]

2 things:

1> When you make time limited software: You provide a different version that's just the demo - killswitch for the final delivery after final payment.

2> You don't delete their data ever. That's their data. It's their work. Nothing can make it your work. Use of the demo to make their work can be an implicit acceptance of any contract making them pay you, but don't ever screw with their data/work.