Via user guide
If you have files in your sync folder that you don’t want BitTorrent Sync to track, you can use .SyncIgnore.
.SyncIgnore is a UTF-8 encoded .txt file that helps you specify single files, paths and rules for ignoring during the synchronization job. It supports ‘?’ and ‘*‘ wildcard symbols.
Note that .SyncIgnore is applied only to the folder where it is contained and will not work with the files that have already been synced.