I believe you'll have to elaborate on what "the oneway channel" is.. When you connect to a server, the server might force you into a channel-- there is nothing to fix in this situation because there is nothing broken. You simply part the channel and go wherever it is you want to go. If the server doesn't allow you to part the channel, then there is nothing mIRC can help you with- that's something you need to take up with the network admins.
- argv[0] on EFnet #mIRC - "Life is a pointer to an integer without a cast"
In addition, if this happens on all networks, you probably have a script loaded that's causing it and you'll have to either remove or disable the script, or edit out the line(s) that causes you to join that channel.