I semi-agree with what you did in this case, and here's why. If you had spontaneously canceled his account, without having stipulated what kind of behaviour yo

I semi-agree with what you did in this case, and here's why. If you had spontaneously canceled his account, without having stipulated what kind of behaviour you find unacceptable in the Terms of Service beforehand, he could rightfully accuse you of being capricious and arbitrary. As a customer, that kind of action on your part would definitely make me think twice before signing up. Even though I'm not a patent troll, what if you some day decided that what I was using it for was also unacceptable? Speaking of the letter of the law, the ToS that you both agreed to didn't mention anything about patent trolling being out of bounds. What I would do in future is maintain a list of un-OK uses as a schedule to the ToS, but grandfather existing clients into the old ToS (perhaps with a 30 day grace period, allowing customers to export all of their data). In the case of customers using the software to do things you don't like, rather than terminate their account, just politely ask them to take their custom elsewhere (with no obligation to do so). I think you'll find that most people would rather be where they're wanted. Caveat: IANAL. You might not be able to do that sort of thing (although I'm nearly sure you can).