Get rid of the option for anybody to unlock an object, but instead allow to unlock only by the person that locked the object or by a group in group role in the database where only few people have access.
Jim Bozich commented
In general, the source control process should be tight and secure. I have seen lock icons where you can hover over the lock icon and see who has it checked out. We are currently evaluating "SQL Source Control" and another companies SQL Source control product. The one major issue I have with your product is that anyone can unlock an object. The other company only allows the person who locked the object and a special admin role to unlock it. It would be great if your product had this feature.