Remove Flag Or Reminder From Message. Choose File Options Advanced. They have already made their booking committed their time and maybe even their money if you use a deposit system.
The first two are to remind your clients to show up for their appointment. Clear the Message Flag rule. So the reminder is essentially a different thing than the Follow-Up-Flag.
It may change after deleting or moving messages.
Meaning you no longer need to follow up on that item. You can quickly flag. On the Standard toolbar click Follow Up and then click Add Reminder. How to remove flagged emails from task list in Outlook.