We were running into that problem a lot, not only with leaves, but with mowing as well. Some customers are out of town a lot, and some just want to get out of paying, so you really need a to be able to say emphatically that yes, you were there.
One thing we started doing was putting out door hangers at every job with boxes checked for the work we did that day.
The main thing I have found that helps is keeping pristine records so that when a customer says we weren't there, all we have to do is show them, not only the dates that we serviced their lawn, but the exact times. If you don't have the time to keep those kind of records, then private message me and I can give you some more tips that will really help you in this area!
In the end, if that doesn't help, then you probably need to drop that customer.