Suppose you have a particles breed, and a separate stopped-particles breed. You could do something like this:
ask particles [
if zcor < -14 or any? stopped-particles in-radius 1 [
set breed stopped-particles
That way, once a turtle reaches the ground, OR gets too close to another particles that's already stopped, then it will change to being stopped.
(If you're doing it in NetLogo 2D, use
ycor instead of