I have a public folder to which all members of our staff group have access. I need to deny access to this folder for one member of the group. Is there a way to do this? For regular folders I can just set a 'deny' permission which overrides the permission granted to the group. Public Folders doesn't seem to have that.
The permission you want is None.
However, most permissive wins. Therefore if you grant a specific user "none" but they can get permissions another way (group membership) then they will continue to get access.
If you want to block the user from that folder then your only option is to create a group that user is not a member of, and grant permissions to that group.