The PPA only has a mono package for Natty, not Maverick. The only package explicitly for Maverick is Pinta.
You could edit your
/etc/apt/sources.list (it might have a separate file of its own in
/etc/apt/sources.list.d/) so the relevant line goes from something like this:
deb http://ppa.launchpad.net/directhex/unstable/ubuntu maverick main
deb http://ppa.launchpad.net/directhex/unstable/ubuntu natty main
This won't always work and might even break things but it's only Mono (I don't think anything critical to booting relies on it) so you could just use
ppa-purge to drop the PPA and go back to stock packages.