If projects or tracks are deletet they are moved to the thrash bin.
It is not possible - as it looks like - to delete these items to save storage.
Only restore is possible.
How can i clean the trash bin by deleting items?
If you are talking about the trash bin in the Cubase project pool then all you need to do is right click on the trash bin folder and choose to delete the contents. Be careful here because once you delete files from the project pool trash bin the files are gone forever. Also always consider if the files you are deleting from the Cubase project pool trash bin are shared with other projects too. Because if they are, and you delete them, then they will be gone from the shared projects too. The “pool” can be very confusing and contain shared project files. Especially if you haven’t been keeping each projects pool clean (so to speak).
So… BE CAREFUL! Empty the trash at your own risk. 
Regards. 
Yes, as Prock said, be VERY careful. Hopefully though, you regularly back everything up on an external drive for safe keeping… just in case you end up deleting something that you were not supposed to delete. 