The most important thing I've ever built was my contracts - and these help set boundaries for these clients. Need something extra? Well that's not in the contract. Let me get you a change order. I think where some contractor's get in trouble is maybe they messed something up in the beginning and feel like "they owe" something to the customer, then start giving out freebies. That's a slippery slope. I was that guy for a while but what I started doing, if I'm giving them a freebie - document it! Issue a change order for $1 for the thing you are fixing, outline in the change order why you are doing it and make them sign it. Then it ends there. When they want something else, new change order.
I've also had people that we've done this with and they still are a pain. Just get through the project you are on and if they reach out again just tell them you are at capacity and suggest someone else for them to go with.
