on the ConferenceRoom networks, and I'm still learning the details of it, if you're not OP'd (or maybe even voiced), then if you get kicked or /hop the room, it doesn't let you rejoin for 5 seconds. I've noticed at times where it still lets someone rejoin instantly, but it's not that common if they weren't op'd.
Something I think I forgot to say earlier is that I personally think the auto-rejoin option is a bad idea because it can be used to flood someone off of the server. Kick someone out of 10 rooms, they rejoin those 10 rooms, send the /names #roomname (mirc does it automatically anyway), repeat and they're flooded off within a matter of seconds.
There are some options that I wish could be altered rather than just turned off. Take, for example, the rejoin room when kicked function, if there was an options area that would list "scripts" for the functions, per say, and allow you to use your own code in place of the default method.
Like, under tools, "Internal Command Scripts" as an example, and for the "Rejoin channel when kicked" option, have a default script that can be viewed, probably looking like this:
ON *:KICK:#:if $knick == $me .join #
And then a bigger box underneath with the option to use it instead of the default coding, so you could design your own rejoin script, or perhaps a more simple idea would be to check a box and then you could tell it an alias to perform (along with $1- or other identifiers of your choice) instead of the default.

That way if you design a more complex but also more reliable room rejoiner script (like I have) then you can turn it on/off in the mIRC options menu, instead of the option just sitting there taking up space.
Ok maybe now I'm just babbling so I'll shut up.