Are you having this problem at the moment when you are physically playing the instrument, or when you play back a track upon which a performance of the instrument was recorded?
If the latter, do the note lengths in the Piano Roll Pane show that the instrument should no longer be playing? (I've been having a problem in 10.2 with recorded takes missing the "note off" events and resulting in the notes in the Piano Roll Pane stretching on and on, beyond what I had played. I'll be posting about that in a separate thread.)
If the note lengths aren't longer than expected, then I agree with Edgar ... look for a stray Sustain On event in your Event List. Or just check for it by playing the track and then seeing if the notes stop playing if you quickly tap your sustain pedal during playback.