There is no 'Inhibit' function as such but I would suggest the best solution is a group master assigned to a fader. This should be the same other than the action will be inverted - ie. at full = assocatiated fixture intensities are unaffected / at zero = associated fixture intensities off. Set to scale for proportional control or limit for a hard limit.
Alternatively you could try an empty cue set to mode 3. Raising this will proportionally fade out intensities from all active playbacks with the same or lower priority.
Finally you could look at a custom shape. See the guide I wrote
here. Aside from the obvious unsupported customisation the biggest danger with this the need for the custom shapefile to be present when loading the show. This is particularly important if you are moving between consoles or when updating the personality library. I wrote this before group masters were implemented.