You can cancel a schedule at any time and you can also completely delete it, just follow these steps:
1. Go to the Activity schedules tab and click on the 3 dots next to the activity you want to cancel/delete the schedule for, then click on Delete schedule.
2. A menu will open so you can decide if you just want to cancel that occasion or if you also want to delete it. If you select cancel and delete all futures then you have the option to choose the days that schedule will be removed.
Keep in mind that if you delete a schedule, you cannot get it back. And your clients will automatically be informed of the changes so you don't have to worry about doing it.