How do I set up the system so a user cannot enter purchase orders if there are insufficient funds?

Go to the System Administration menu -> Control File Maintenance Menu -> System Parameters Menu -> Define Site Control Parameters (LG0144).

There are two ways you can do this:

Option 1: Click the 'A/P' button (LG0541) and mark 'Place Invoice on Hold for Insufficient Funds'.

Option 2: Click on the 'Budget Ctrl' button (LG0587) and uncheck where it says 'Allow Over Expend'.