Though we love collaborations, we are aware that there might be times when it just doesn't work out.
In order to remove a collaborator from your project, from your profile page, open the dot-dot-dot menu and click "Collaborators" (1). Click "Remove" next to the collaborator(s) you wish to remove (2).
There are also a couple of things you could do before you start a collaboration in order to feel more safe about your project. Make a copy of your project by going into the File menu in the studio and choosing "Save as..." If you are a Premium user, you can use Time restore to open up a previous version of your project and create a backup even after you've started a collab. If you change the name in the studio, the next time you save it you will be asked if you want to "Save as new". Thank you, Captain Hindsight!
Good luck! :)
(1) Profile page - dot-dot-dot menu - Collaborators
(2) Collaborators dialog