That's a limitation from the way the advertisement is positioned.
The "guides" and "What's new" also don't seem to be able to go to the absolute bottom.
Anyways, you can remove the advertisement. Most skins have an "advertisement removal" .reg file deep within their folders (at Program Files/Messenger Plus! Live/Skins/<skin name>), just take a look there, apply it, and restart messenger.
[If Messenger Live Toolbox doesn't have it,
My Live can also remove the scene.]