In situations where a merge is required, the files will be listed in the Synch Merge area. You may view and edit the files that are undergoing a merge or check out files and manually reject some of the changes brought in by the sync. When you are done reviewing the files click on Accept Sync.
NOTE: You cannot simply refuse to accept a sync, because it's already been sent to other members of the project. All you can do is to make your counter-changes and check them in after accepting the sync (or restore the project to its previous version).