How to prevent posting to labor g/l accounts by non-labor processes?

We are attempting to prevent posting to labor accounts by non-labor batches.  Since the account has to be available for posting to charge time cards to it, we've been unsuccessful finding a way to restrict A/P users (for example) from entering items that will post against a labor g/l account. 

Is there any way to do this without customizing E1? 

If you customized E1, where did add the modification?