If you have used the Cinnamon ppa for Precise available at Launchpad, rather than the Mint repositories, it shouldn't have altered your LSB information, although I reproduce the 12.04 LSB information further below.*
The useful program,
ppa-purge, is in the repositories for 12.04, but you have probably tried that, but it is useful as it removes all the ppa's files and dependencies and not just the ppa's list.
I assume your main
/etc/apt/sources.list is functioning and you have checked that no references to Maya have crept in there and that there are no remnants of the Cinnamon ppa left in
Nevertheless, if you suspect your
sources.list has been contaminated, you could run
sudo sed -i 's/maya/precise/g' /etc/apt/sources.list
which would substitute any references to Maya with precise, and was useful in this different case at Ubuntuforums.
*However, for your reference, this is the output of
/etc/lsb-release for Xubuntu 12.04.1
DISTRIB_DESCRIPTION="Ubuntu 12.04.1 LTS"
If you were running the original 12.04 and haven't upgraded to the point release, 12.04.1, instead use
"Ubuntu 12.04 LTS" and not
"Ubuntu 12.04.1 LTS" in the final category.