It's a civil tort -- lawsuit time if its worth the expense to you.
Work For Hire
Depends on the contract that you made.
Always get a transfer of ALL copyright to you with the invoice that you pay for the work done [ 'a work made for hire' ]
Trying to enforce Promissory Estoppel and Detrimental Reliance in a lawsuit is a legal pissing contest if the contract and the copyright assignment are not in writing.