To add or deactivate an user you need to have Admin rights. If you have admin rights, you will have the "plus" button visible on the Collaborators page or also you can enter yourdeploymentname.valispace.com/admin. If you automatically log in to the admin panel, then you have admin rights.
To deactivate an user, you need to go to the Collaborators Page and add the Active column, by editing Visible columns. To deactivate, you can uncheck the box of the corresponding user. Refer Admin Features for detailed information and gifs