How to disable PHP "exec" & co. server-wide on the OS level

[Bitpalast]

Security tests have shown that without a disabled exec command in the php.ini settings of each account customers can easily read-access files like /etc/passwd and manipulate other files on the host machine.

We would like to eliminate the need to have a disabled_functions=exec... listed in the individual php.ini settings. Can this command be disabled for all PHP instances regardless what a user enters into his php.ini settings?