Boards have a "share" button that users can use to copy the link and share it with specific users. It is not always desirable to have that allowed, so the suggestion is to have a way to disable that button.

h4 Workaround
The known workaround for that is to remove the user from the global permission "Browse groups and user". That, however, won't actually disable the button but will show a message saying the user cannot share. If the button was disabled for the user, it would not be needed to click it to find out it is not an option.