Hi there!
I have a little piano piece in Renoise and I want it to sound as if its cut off after a certain note.
The effect I want is pretty simple, I just want the note to sound and the next line I want just total silence, nothing more, nothing less.
I tried the Note OFF and the FF00 commands but the notes just keep playing silently 
Its the exact effect you get by just pressing spacebar as soon as the note has played.
I AM using a vst (mda piano) could that be the problem?
In mda piano VSTi external editor drag the “release” slider to the full left and re-use OFF command. See if you get the desired result.
Ohhh thats brilliant!
Thanks mate! 
alternatively, you could just drop a Gainer on the track and automate the gain level (jumping between the desired level and zero). if used properly in conjunction with note off messages, this could quickly cut out all the sound on a specific track.