Check the help under Particle Collision Events to be sure, but I believe that if you create a particle collision event you can just leave the three checkboxes for "Emit" "Split" and "Random # Particles" off.
Then just set the "Num Particles" slider to 0.
Try that, and let us know if it helped at all
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com