Customize AR invoice memo screen

I have a client on SL2011 SP1.  They have Hard Close and ASI AP to AR third party apps.  I am trying to disable the AR account and AR sub on the Account Override tab.  I exported all customizations to take the screen back to the standard screen and tried disabling the fields with no luck.  The field will show it is disabled in Customization Manager but when I go to enter an invoice the fields are available.

The client wants to prevent users from putting in different values in these fields.

Any help is appreciated.

Thanks,

Cherie Kane