I've been trying to program the reverse thrusters using a button on my Saitek Throttle. The task I pick is  "  decrease throttle " . Oddly that works, but I can't find the right task to cut them off. I have exactly the same problem using my keys.

I assigned a button on my throttle quadrant via the buttons and switches tab.
Select for FS control - Checked
Control sent when button pressed - Throttle decreased
Control to repeat while held - Checked
Control sent when button released - Throttle cut

Works fine for me in the NGX. Adds reverse to both engines whilst button down and sends throttles to idle when released. Never failed me yet.
If you want seperate reverser controls for each engine assigned to 2 buttons then select Throttle 1 and Throttle 2 instead of Throttle.

Thanks so much Plate, I never would have thought to use  throttle cut.

+1 (or 2 if you want) on graham's advice !!

Here is my code in FSUIPC:

 

18=R1,6,C65602,0 ;        With next, thrust reverse - repeat trottle decrease

19=U1,6,C65604,0 ;        With previous, thrust reverse - trottle cut command on release

 

Controller 1 is my throttle quadrant, button 6 is the detent button actuated when you pull the throttle below the zero point.  (command line number will vary but you can skip lines / omit lines as long as the lines follow in order)