That's rough buddy. One way you could stop that in the future is the bookmark feature. It allows you to make a copy of a song without remixing it, so you can make a bookmark to make sure something isn't destroyed. Pretty sure Jordi Moragues explains it way better than I do. Try searching the boards for bookmark, that will be a better explanation than what i have here.
If you're the owner of the track, you should be able to go to the Invite People screen (Accesed by using the icon in the top right of the editor) and clicking the X button next to somebody's name. If you have Admin rights you should be able to kick anybody with lower permissions than you. Good luck!
how do i search up accounts