Cross-Forest migration after the fact

Hi All,

Hoping someone can't point me in the right direction.  I have a user mailbox in forest A who has been migrated to forest B using the cross-forest migration process.  I now wish to prevent that user from dragging emails from their mailbox in forest B back to any mailbox in forest A, there is a one way trust setup where A trusts B.  Without touching permissions is there a group policy that explicitly would deny that type of copy request? Something along the lines of no user in Forest B can drag and drop email back to a mailbox in Forest A.

Thanks,

Tom