Hi and welcome to Arturia forums.
There is'nt any "note off" trigger.
But modulators can be set to have both negative and positive modulation impact on any parameter including as a sidechain. Have you tried to experiment with that?
If you in example:
1. turn up the Sample/ Grain volume fully
2. Add an envelope as a modulator with a fully negative value for that volume
Then you will get no sound when the modulator envelope is in the sustain phase, if the sustain is fully up.
When you release the key, then the volume will turn up according to the set release phase.
Perhaps what you wan't is that simple. But you can modulate other parameters to get different effects.