[Mehmet]

Powers p. 154; it is state that "voluntary deactivation is only possible with switchable powers" so I'd say you'd need (I know, it is a bit extreme :)) an affliction or something that NEGATES your own transient ability. I wouldn't though :) If it is not too unbalancing I'd allow early cancellation of transient abilities if it has "variable" enhancement (OK, if you wish, we may call it "endable" :))...

Cheers...