When using SuPHP on a server, a customer can create their own custom php.ini file that will allow them to change the PHP settings. Which may or may not be a good thing depending on what they do.
You can stop them from doing this by disabling this feature.
To do this, SSH to the server, and edit the following file with your favourite text editor.
Once you have it open, search for the following lines, and uncomment them (remove the ; from the start of the line)
Save the file and restart apache. Your customers will now be forced to use the server php.ini, and can not make changes by using a custom php.ini.