Allow to disable a new button to a group of users

Hello,
For some employes, we need to disable the possiblity to create a new category for partners. We already remove the menu entries under Partner/Configuration but now, when they add a new partner, they still have the possibility using the form 'Partner Category' to add a new category. So, for these people, the idea is to deactivate the button "new" ; it's cleaner to do that than allow them to create the new category and then displaying an error message when they try to save it.