I think the Requirement type Folder should never be allowed to have test coverage. This is the default in ALM.
If users feel it is sometimes needed you could create a special folder type as a copy of Folder and allow that type to have test coverage.
That approach would not work for preventing users from adding every Test under the Subject root to a Requirement. But you could perhaps decide on some maximum of Tests allowed to be associated with a single Requirement. Then in Req_MoveTo you could check the number of associated tests using GetCoverList. If the limit is surpassed you could for example send an automated email to a project administrator.