What controller?
The keyboard command is "K". The motor will start again if you drop the stick all the way down, and back up again.
Easy to do if using a fully programmable radio running OpenTX, EdgeTX or ETHOS. Maybe (I have no experience) with a top end Spektrum.
I did not take the time to figure it out, and only have a very old Interlink (a non-DX) - but it looks like you could set up a condition using an unused channel to drive the throttle to fully closed using the software radio. However, this would only be specific to a copy you made of one aircraft, and not applicable to any others directly.