To prevent non-administrators from adding/editing/deleting records, open web/staff/app/cls_ps_(--table-name--).inc.php and add

if ( !$this->sys->IsAdmin() ) exit;

after

case 'edit_inp':

The result should look like:

case 'edit_inp': if ( !$this->sys->IsAdmin() ) exit;

Do the same for the following lines, too.
case 'edit_done':
case 'reg_inp':
case 'reg_done':
case 'del_multi':

How can you print a message saying they don't have access ?

You can print some message before exiting like this:

case 'edit_inp': if ( !$this->sys->IsAdmin() ) { echo "Access Denied"; exit; };

Please change "Access Denied" to anything you want to display.