MSN is difficult to uninstall esp. if you are using XP.
If you go to Start/Run and type RunDll32 advpack.dll,LaunchINFSection %windir%\inf\msmsgs.inf,BLC.Remove
MSN is uninstalled from your system.
Does not stop it getting reinstaled by anyone with access to your computer though.
Perhaps there is a firewall package that you could deny MSN access to the internet? You could do it with zone alarm but I guess you would want to be able to set up access rights to the firewall so no-one can change the settings.