command restrictions
 

how can i make su available only to certain users or user groups? same with other commands.

As for su, don't give anyone the root password. There is a software package called sudo that will give limited root capabilities to various users with them needing to su to root.

the question wasn't if anyone knows the pass or not. just can i disable it from some users or not.

You could try changing the group ownership of su, setting the appropriate permissions, and then create a new group with those users.

Whatever you were asking, disable su seems a fairly futile exercise. Whatever account someone logs into using su they must have the password for it, so if you disabled su they could simply log in to that account to begin with.