i think a buff is mainly the visible effect of a spell...and is there for the duration of the spell...so when you spell steal and the buff transfers, that's the signal that the spell was stolen. you can always make a dummy unit cast silence at a point. that could be a little complicated...you could also use soul burn, since that silences also. doom silences also. but both of those have alternate effects than just silence. so you'd need to edit those parts out if you use it. but both of them can be cast on targets whereas silence casts on a point. doing a point you could silence more than one target at a time.
anyway i suggest using a dummy with a modified version of silence or soulburn or doom.
careful with doom, even if you make the dmg per second be nothing if you kill it while silenced i think it will come out with the spawned unit. of course you could always change the spawned unit to something harmless such as a critter.
following what i said at the beginning of this post...with stun you'd need a modified version of a spell that stuns to be casted...most people use storm bolt i think...