When using the /mode command you must specify the nickname the same amount of times that you change the mode for a user...
So if you set mode -eo on a user then you must use /mode #channel -eo nickname nickname because you have changed TWO modes. So in your example, use:
/mode #channel -meo LeBoOo LeBoOo
and all should be fine. If it's still displaying the user as opped then it's more likely to be a bug in the IRCd and not with mIRC.
Regards,
