Type /!remote off to prevent any scripts running (though this won't prevent STARTUP scripts because you can't type it before mIRC starts up).
If you have a script, then I would say it's actually that forcing you to join a channel - check out the channel name and topic - if it's related to the script, then it will be the script, if it's something else, then it may well be a network channel. Also, if you type /!remote off and still join then it's the network.
Don't install scripts if you don't know what they do, or how to control them.
Regards,