“Disable activity” option is available on right click menu over already disabled activities.
I would expect not to get that option in an already disabled activity. All you get by clicking on it is to expand the amount of nested “disabled” layers, that sometimes get 2x extended (adding an “Ignored activities” extra workflow I don’t know why):
Gosh, as there are some different “levels” or “layers” or “nests” in the affected activity, sometimes you may find one which offers BOTH possibilities: both “Enable Activity” and “Disable Activity”, all at once, which, obviously, doesn’t make any sense: