Unfortunately, SQL Source Control does not put any kind of lock on the repository to stop users from making changes from outside of the tool. As access is determined by users permissions (which is independent of the tool), I cannot see how this could happen using the current model.
However, I would recommend that you post on the SQL Source Control UserVoice Forum. The development team use this to gather feature ideas and to gauge user interest in them. Please include a brief description of why this would be useful.
http://redgate.uservoice.com/forums/390 ... ce-control