That tag is persistent across the sync in order to maintain the date added regardless of the source to which it was added.
Best thing I can suggest (if you haven't opened/closed rekordbox more than 2-3 times) is to go and re-load one of your database backups prior to you adding that playlist.
Go to: ~/Library/Pioneer/rekordbox
Find the datafile.edb, rename it datafile.withitunes.edb
Find datafile.backup2.edb (because chances are the datafile.backup.edb also contains your iTunes imports, but check it against the time/date of last modification to see if it was before your import) and rename it to datafile.edb
Load rekordbox and check to see that it doesn't have those tracks.