Yes, actually more and more servers support the +r flag in channelmodes to prevent users from joining the channel unless they are registered on that network.
For example on irc.gamesurge.net you must goto their site
www.gamesnet.net and register an account and then in irc auth to authserv before you are allowed into +r channels.
(Most irc servers have sites with information and help btw, just replace the irc in irc.xxxx.xx with www and open it in a webbrowser).
The most common reason for this on gamesurge is that they suffer from a lot of kids that get angry and use botnets to attack channels, a botnet is a large number of fake users that join the channel and spam it etc., obviously when a channel is +r the fake users who did not register cannot join the channel and so these attacks are prevented.
It would be nice incidentally if mirc supported the many modern channelmodes that have come around in the channel central window, at least the common ones like +r and +c (no colorcodes) etcetera.
And it would be even more convenient if mirc had a warning/explanation on rejection of entering when a channel is +r, many many people get confused by this channelmode in my experience, and I'm given to understand that it's the clients like mirc who must handle errorreporting.
