This is a case of 7H and how it functions. If a new mod or update is added, everything is automated and the user just clicks the Download button. It doesn't work the same in reverse because that was honestly something Iros didn't think to code in. So when a mod is removed (in this case, SSM is removed), it still remains in your catalog xml found in the ..appdata\Roaming\7th Workshop folder. If you close 7H, delete that catalog.xml, and reopen then you should see it gone. Also, any users setting up a new 7H after the removal will not see the removed mod in their catalog.