This thread in ubuntuforums solves the problem with synaptic:
http://ubuntuforums.org/showthread.php?t=1685945

Go into synaptic

Select the "origin" tab in the lower left corner. There you can check
if you have any installed packages from a specific repository.

If you find any repositories you don't need anymore you can go to
"software sources" on the "administration menu" and remove them from
there.

If you don't want to remove them completely, just untick the box to
disable them.

I would recommend you uninstall any packages from a repository you
want to remove since you won't receive any more updates for them.