Is there a 'panic button' in Psycle that kills immediately all the sound?

Yes and no.

It isn't on the main toolbar, so it isn't a button, but you have the menu option "Enable Audio" in the "Configuration" menu, with the associated Alt+A hotkey.

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<[JAZ]> Pa pi pa pa pa pi pa.... ;·D

ALT+A only PAUSES playback.

I want to kill playback of long sample (or synth + long delay effect) which is still playing when I stop (F song.

(nothing to do with note-off command)

(fukkin emoticons)

ALT+A only PAUSES playback.

I want to kill playback of long sample (or synth + long delay effect) which is still playing when I stop (F8 ) song.

(nothing to do with note-off command)

Both, Sampler and Sampulse should stop samples once the F8 key is pressed (with a delay of at much 3 milliseconds to avoid abrupt sound stop).

If it's a VST, then it's up to the controller (we don't send a reset to the instrument because that could have other effects).

Usually, the problem is with effects, reverbs with long tails and such. What I used to do with those cases is to mute the effect, lower the volume on that machine and wait for it to stabilize.

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<[JAZ]> Pa pi pa pa pa pi pa.... ;·D

waiting forever...

workaround:
delete connection, re-connect.

workaround:
instruments - replace machine with another instance of the same machine.
effects - reconnect or simply click BYPASS button twice.