Protect su with pam_wheel:
=========================
You need to add a new group "wheel" to your system.
# addgroup --system wheel
Add users that should be able to su to this group.
# usermod -a -G wheel user1
Then add the following line to /etc/pam.d/su:
auth requisite pam_wheel.so group=wheel
Other users will not be able to become root, they
willget a denied message if they try to become root.
test1@deby:~$ su -
su: Permission denied
If you want wheel members to be able to su without
a password add the following.
auth sufficient pam_wheel.so group=wheel trust use_uid
.