Probably a bit late with this reply but you can cancel an AIB SO online using 24 Hour Banking, just click the correct tab when you log in and you should see all the SO you have active, simply hit the cancel button (it will ask for a digit from your login code), simple as that.
But if your rent is being reduced (and still paying to the same account/landlord) then you can simply just alter the SO amount -you don't need to cancel it and begin a new one - again you can do online in the same manner.
I know this as I have many SO on my account and I change the amount on these nearly every month. Really easy and quick.
As said above if its a DD then you can not do this online for AIB, you will need a letter (post or fax I believe - soemthing with a signature)