Prevent G/L expense accounts from A/P

Hi,

I would like to have your help on the following issue:

The customer has activated non stock procurement for services and some other expenses and needs to prevent the users to enter the corresponding vouchers directly through Accounts Payable. He needs a type of control and if the users by mistake try to update the non stock expense accounts directly through the A/P, instead of following the procurement flow, then an Error message to be appeared.

Has anyone experienced with something like that?

Have you got any ideas?   

Thanks in advance

Lia