You can reset all settings to default by deleting mirc.ini while mIRC is closed. It will be located either in your mIRC install folder or else in your AppData folder. The easiest way to get to that folder is to go to the Start menu and either choose Run (XP and older) or type in the box (Vista or 7) and run: %appdata%
Then open the mIRC folder and delete mirc.ini from there. When you open mIRC, it will recreate the file with the default settings.
Otherwise, just delete everything under Proxy in mIRC's settings and set Connection to None.