I agree with Lesli. There is no reason it should be tolerated and dealing with is ASAP before it gets too far is best. Wronged customers can get rather irate, though, so if there was a problem on your end and they're rightfully upset, then simply dropping them without warning isn't really a good idea....but you should still try to keep it under control and do your best to fix the issue and do some damage control, even if the customer seems 'too' upset.