Thanks for your bug report. I have not been able to reproduce this issue. I tested this by changing the setting in the Options/IRC/Events dialog as well as in the Window's system menu Event dialog. In both cases, quit messages were hidden from the channel.
If this is not working for you, you may be using a script that displays custom quit messages.
If you install a clean copy of mIRC in an empty folder, using the "portable" option in the installer, without any scripts, and test it, does it show the same issue?
