If you want to remove the banner adverts from the non-split-mode message windows you will also need to reduce the height of dialog resource #2503 to 156 and #2507 to 164. This has been tested with ICQ 2000b build #3281.
Sometimes the banner ad on the outgoing message window appears, but underneath the other buttons. Even if this happens to you, you still save on the screen real estate that the banner ad used to gobble up.
Remember - back up your IcqCore.dll file before messing with it. If you screw up totally then you'll have to reinstall ICQ, so make a copy of your settings first.