Are you sure its a MWI that has been set by Unity or CUCM? I know PABXs also have the capability to set MWIs regardless of whether there is a voicemail system or not. Easiest way would be to use your Avaya PABXs code to turn the MWI light off.
If thats not possibe, dial 1002 from the affected extension into CUCM.
Hope this helps!