Verb to describe "to enable or to disable"
May I suggest "to set availability"? I think it fits well for function names: SetButtonAvailability(), SetOptionAvailability(), etc. It's long, but descriptive...
The verb form of toggle can be used to indicate changing state
[no object, with adverbial] Computing switch from one effect, feature, or state to another by using a toggle:
the play/pause button toggles between those functions
[with object]: there are a number of attributes which can be toggled on or off
The verb to set just jumped to mind.