It is most probably defined in more than one place.
If it exists in your design, it will be in the design assignments.
So,
open pcb design and delete from assignments.
close pcb design.
open library editor and delete from assignments in library editor.
If it is used on a part, open part and delete from part assignments.
Then maybe? it has gone.
If you use a part that uses the pad, it will re-appear!
Cadstar's like marmite, you either love it or you hate it!