Yeah it's really annoying with how shitty the ban function is on MFC. Here is how I ban.
After you ban them, go to the options menu and extend their ban to lifetime. Otherwise they are only banned for a few hours. Also ignore and mute them (mute only lasts a few hours), which prevents them from bitching to you in PMs.
You can ban them from viewing your MFCShare as well, if you go to your Share settings, there's an option automatically ban members from MFCShare if you ban them from your chatroom.
MFC is really not the best when it comes to banning, I dont see why they dont just update it.