There are 1000 steps you can take before it would come to using a back door at all. Formulate a contract, agree to exact terms, etc.

Remember what it was like when we first started out. No clients, just needing to take any job to keep the lights on. You expose yourself to an array of clients, some upstanding, others not so much. If I felt like a client was dodgy, avoidant, refusing to pay or consistently trying to move the goal posts, yeah I’m putting in a contingency plan there. If you get paid then you remove it.

So how do you avoid getting sued? Before you start, just write this down:

“The developer will retain full control of the application including the ability to shutdown the application remotely should they not receive full payment, after which time such contingencies will be removed”

Doesn’t have to be a secret.