If you are just wanting to disable PHP completely for that website then what azrael suggested will work. That account will get Internal Server Errors any time a PHP script is accessed, but it will stop PHP from being processed.
If you want to disable suPHP for a particular account, but still allow PHP to work, then you would have to have mod_php installed as well. I don't recommend running both mod_php and mod_suphp on the same server. I think you are just going to run into problems.
The bigger question might be to find out why you want suPHP disabled for one account and what can be done to resolve that situation.