A decade ago I had a client that I registered a couple of domains for with the intention of setting up a website. At the time, I registered them in my name (just the way the registrar worked at the time, I know that's bad practice).
The client then spent 6 months making me work nights and weekends setting up and making changes to their site. They never paid me a dime, and I'm pretty sure they never intended to. (this was at the very beginning of my self employment and I was trusting everyone and going way above and beyond). I eventually took the sites offline and refused to transfer the domains until I got paid. I am still angry at the amount of time this person took from my family life, with no intention of paying me.
I never gave them the domains, and I've kept them registered for 10 years. the domains are worthless otherwise.
At what point would you stop renewing? Bear in mind that I'm still very angry at my treatment even now - and I don't normally remember this stuff past about 2 days. The only thing that could make this worse is if I dropped the domains and they picked them up.