I haven't tried this, so I won't swear it will work. But you could change the permissions of /usr/bin/su so that only members of a specific group could run the command. For example, since su's group is sys, make the users you want to run su secondary members of the sys group. Then remove world execute for /usr/bin/su.
Adding these users to a privileged group might bring up other issues, so perhaps removing execute perms for others and creating an ACL to allow members of the ACL to run the command would be a better way to go.
Larry