Here's what you do, cuz I made that same mistake early on. Don't mulch, keep bagging.
Do not let these people dictate when or how often you do it. You set a schedule? They agree? Then that's the schedule. They want to skip? You start charging by the hour not the job. I had a customer try and hose me by making me come AFTER hurricane sandy to bag up her leaves. I said yeah that's fine, but I'm canceling the price we agreed upon now that you're changing the schedule.
I'm a real nice guy, am friendly with all my customers, and have a great relationship with them. However, I do not put up with BS and I kindly let them know this from the start. Helps avoid these situations.