When the reminder pops up click on the dismiss button. If you are closing the pop-up it probably is doing the same thing as snooze and will keep reminding you of the appointment. This will keep going on until its dismissed.
In Outlook 2010 and 2013, reminders can be turned off in File, Options, Advanced.
Say you return from vacation, and have 100’s of reminders.
If you forgot to turn them off before you left, you can turn off reminders, then close and reopen Outlook.
All past due reminders will be turned off (dismissed) for you when Outlook reopens.
Just don’t forget to turn Reminders back on for new reminders going forward.