Command for taking out on-shell particle.

Hello John,

Do you mean to forbid events where a given s-channel propagator is onshell? In that case you use the "$" formalism, e.g.
p p > sq sq j $ go
forbids onshell s-channel gluinos in the process. Note that this is complementary to using the decay chain formalism to include the particle - i.e.,
generate p p > sq sq j $ go
add process p p > sq go, go > sq j
is (almost) equivalent to
generate p p > sq sq j
("almost", because non-resonant interference diagrams are not present in the decay chain, which is fine since they are suppressed by the near-onshell approximation).

If you mean to completely remove any diagrams with a particular s-channel particle, you can do the same but using "$$" instead of "$". Note however that this is in general not gauge invariant.

A third way is to forbid any diagrams with a given intermediate particle (s- or t-channel). This you do using "/":
p p > sq sq j / go

Finally, if you just mean to remove the propagator line from the event file, then I would suggest that you simply write a script to manipulate the event file in whatever way you need.

All the best,
Johan