Forum Moderators: not2easy
Thanks in advance and sorry for my bad english.
So is the person who submitted, but that does not help you.
Especially as you KNOW that the stuff is not 'public domain', and you could never convince a court that you did not know what was going on.
You might get away with it if you ask your submitters to sign a form "under penalty of perjury" - but I doubt you'll get many forms, especially as you'll need to confirm every senders identity.
Bottom line is "stolen property is never yours".