There are worse customers, to me, than those that simply aren't polite. For example, a few times a year, someone will show up, subscribe to a webapp for months, then send an e-mail asking to cancel and get a refund for all their past payments "because I meant to cancel before" -- when they were logging in and using the service regularly the entire time. Even the most polite ways of saying "no" seem to lead to "I'll just call my bank for a refund then". A series of chargebacks, even if they're easily disputed, is more of a threat than the lost revenue -- and running into people so unethical messes with my head more than a "fuck you" ever would.