Sync Manager issue with external USB key when USB is full ? how to clean it ?

hello how can i clean my USB key when i have too much tracks sync via my Rekordbox sync manager ? i've tried to stop synchronizing some playlists but it does not work even when i go to the minimum, Sync Manager is not deleting the tracks on the USB key. it seems that the tracks that are no more synchronized are still stuck on my USB key. Is there a way to erase / delete the non synchronized playlists/tracks ?

here is the message in French i have " Pas assez d'espace disponible sur le dispositif " 

thanks a lot in advance for your answer / advice.