The event Unit stops casting is fired when channeling stops, whether interrupted or the duration is successfully completed. The event Unit finishes casting fires only when the duration completes. Alternately, you could periodically check whether the unit's current order is still the order of that spell.
Hopefully you can see how to use those in your spell, but if you need more help, we'll need to see your current code to be able to advise you further.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.