You can do everything right and almost always lose. Sorry, just consider it cost of accepting credit cards. :-( Perhaps do an analytics on those that initiated a chargeback to see if you can detect a pattern and then watch out for that pattern and offer to have folks cancel their accounts.

Let's say people sign up, never use your service then file for a chargeback. You can contact everyone that signed up and don't use the service and tell them to cancel since they are not using it. You have to do the math. Will you be losing more money by doing that or by CB fees?