Let me first mention that you should carefully read the roommate release form you refer to, which must be a form created by the property management company, and may describe the conditions under which the release was granted. It is possible that some of the terminology in there might be helpful.
Second, you cannot legally retain her personal belongings as some sort of collateral for payment of a debt to you.
Third, your best recourse for collection, unless you can reach agreement with her for a compromised payment, is small claims court. The process is inexpensive, convenient and locally handled in the Justice Court. The clerk would have the necessary forms for you to complete. Further, no attorneys are able to appear for clients in small claims court.