I'm quite angry on my server project. It has over 1.5 GB, since nothing that ever got there was ever removed. I went in the Projects(Server) folder and deleted everything there was there, but when I connected to the server again it was still full with the projects filled with erased elements.

Erasing elements from server is a damn important....
but until we get that feature, how can I manually delete the project on the server and still have the local project? So I can get rid of all the erased elements i don't need?

1. disconnect from server and close server app
2. delete server projects folder on server
3. in client app use erase removed elements
4. start server and connect client to server

Right, so 2 - delete projects folder on server

- that means I should delete my main projects server? That is... all my work? So I have to get myself a big backup before doing this, right?

its as simple as greg told you. if you have newer files on your client, they will get synced. delete project from server, recover by copying your backup on the server (working version) and restart server or just sync it from your client.
To avoid this, you should remove them always inside the editor and then click the button "erase removed elements"