We realise the "Leave" button isn't ideally placed at the moment - and this will be addressed as part of V3.2 which should appear towards the end of April.
In the meantime my suggestion on the thread Petrush linked to works for all users.
Your final option is to edit collaboration-title.get.js to add a new model flag (e.g. model.canUserLeaveSite) and use that as a visibility switch in the Freemarker template.
Thank you Petrush I follow the instruction of your post and it work. It could be usefull in the future to have something as a permission set that let to configure this. Thank you bye stefano