Hi there
I think that i have had the same problem as i was using ical (i prefer it to outlook for mac) and after i started playing around with an active directory my ical event reminders wouldnt stop reminding me even after dismissing them, anyway
The way that i found to get rid oth this problem is first quit ical, then make sure that ur calander is fully synced with some serveice (for example icloud) then go into ur libary folder (normaly hidden so you'll have open terminal and copy this line in:
sudo chflags nohidden ~/Library/
(it will ask you for a password (i think this has to be done on an administrators account, not sure as i do not have any non admin accounts on my computer))
then ull be able to see it in ur home folder)
Then go into it and you should see a folder called calanders, go into that and move all of the contents into a new folder and move that new folder to your desktop (or anywhere else you want, this is a back up of what was there before) (once you do this there should be nothing left in the calander folder in your libary folder), start ical and it should the pull down all of your calander events from the service you are using,
Then once you are all done and are happy with it use this command to hide your libary folder:
sudo chflags hidden ~/Library
(you will have to put in your password again as you did before)
(this step is optional as if you want to keep your libary folder un hidden just dont put in this command into terminal, as some people like to have it unhidden like me)
If this does not work and you want to go back to what you had before just go back into that libary folder and remove any new folders and put the old folders back (make sure ical is quit before hand as this may mess things up)
I HIGHLY RECOMEND BACKING UP FIRST, and I TAKE NO REPONSABILITY FOR ANY DATA LOSScaused, DO AT YOUR OWN RISK, as i said earlyer i am only letting you know what i did to fix it and so far i have found nothing that has messed up.
Hope this has helped
Thomas