Am I able to disable PHP functions on a user level?
Unfortunately, you would not be able to disable PHP Functions on a user level. PHP states this in their documentation:
Only internal functions can be disabled using this directive. User-defined functions are unaffected.
You will only be able to disable the PHP Functions on a global level. You would find this ability in the WHM >> Software >> MultiPHP INI Editor.
Here you will click the Editor mode, and you will look for the option disable_functions in the editor. With this done, you will then be able to dictate which functions you want to disable for any PHP version you have installed.
You can review the editor further and its functionality here:
You would also be able to review the INI directives here: