How do I exclude intermediate onshell resonances in event generation?

You can exclude generation of events with a particular onshell resonance in event generation using the "$" syntax, forbidding the particle to go onshell (i.e., within +/-bwcutoff*width from its pole mass). This means that it provides the complementary information to what you get from the decay chain syntax, which instead requires the particle to be within +/-bwcutoff*width from the pole mass. (bwcutoff is given in the run_card, and is by default 15). An example would be
p p > sq sq j $ go @1
and
p p > go sq, go > sq j @2
which add up to reproduce the invariant mass distribution of
p p > sq sq j

Note that this means that all interference effects from the offshell diagram are retained in process @1 above, which can be crucial for gauge invariance. An example is
p p > w+ w- $ z, w+ > l+ vl, w- > l- vl~
where the result is completely wrong if the s-channel Z diagram is removed, since it is necessary by gauge invariance. Another example is
w+ w- > w+ w- $ h
where the offshell effects of the h are necessary to unitarize the longitudinal ww scattering.

Also note that $, like other special syntax (> >, /, $$, coupling orders) applies only to the process part where it is given. This means that it can be safely used as e.g.
p p > go sq j $ go, (go > sq j, sq > j n1), sq > j n1
where only onshell go in the core process pp > go sq j will be forbidden.

Finally, if you for some reason want to remove all diagrams where a given particle appears as an s-channel propagator, there are two possible syntax options:
w+ w- > w+ w- / h
which forbids any internal h lines (including t-channel ones), and
w+ w- > w+ w- $$ h
which removes any diagrams with h in the s-channel (this corresponds to the old "$" syntax before MG5 v. 1.4.3). See the UpdateNotes for v. 1.4.3.

All the best,
Johan