[MasterEric]

I see nothing in the rules against it.
One interesting bit though is:
An opponent ROBOT that contacts, but fails to stop a BALL from going over the TRUSS or in a GOAL has not caused either of these actions and does not invalidate the SCORE.

This means that if a blue robot throws its blue ball, and it hits a red bot, but still goes over the truss, its still a blue score, not a red score because the red bot touched it last.

In a similar situation, if that blue robot throws its ball, and it hits a red robot and bounces away, and doesn't go over the truss, it's not counted because it didn't go over.

So yes, you can block a pass.