They can be vindictive, in simply not renewing your contract for whatever childish reason, or refusing to give you a release letter.

If they haven't fired you in the course of your contract, I cannot see how they can refuse to give you a release letter. By allowing an FT to complete his contract, they have no justification whatever for subsequently refusing the issuance of a satisfactory release letter. Fair is fair, and you can argue about that very strongly. Best to give them the impression that you're no pushover well in advance of your contract completion date if you feel they might be bold enough to attempt to pull a dirty trick like that on you. Heck, you can even threaten to name and shame them on the Net if they won't co-operate with you! I doubt very much that will ever be necessary if you show them you've got balls at the first sign of trouble. I'd be very surprised if they don't take the very strong hint you've given them and back off.