Locking Header Account on custom form

I want to know if there is a way to lock down the header account on a customized form.

Why?  I am trying to use separate accounts for accounts payable and employee expenses.  The accounts payable does not present a problem because only my staff enter AP, and I can train them to get the right account.

But... because we are allowing field administrative and managerial personnel to enter their own expense reports (there is a lot of turnover so proper training is a fantasy) - I get all different account numbers populating the header account field - it appears the only way to get the correct account is to give them accounting approval in order to allow them to change the account - which is needless to say also very problematic.