We are working on TFS 2015 and Visual Studio 2015.
Query: Is it possible to stop association of code check-in to a work item after it is marked as 'resolved' in TFS 2015?
In a custom work item workflow, we want NO change against a work item once QA approves the work item for production movement.
No. This boils down to a user discipline issue.
You can enforce it within Visual Studio (and only within Visual Studio) via a custom check-in policy, but check-in policies are relatively easy to circumvent and definitely will not cover every scenario.