I noticed some ops can do a mass join on a certain user.
Is there any script which can block this from happening? I end up having to close the process mirc.exe.

What do you mean? They make you join lots of channels?

If so, all you can really do is stop using a network with immature opers.

Sounds to me you were flooded by childish ops who aim for users to mess about with. As the responder suggested above, find another promising, reputable network to continue your IRC enjoyment.