No its under Setup > Accounting > Accounting Preferences. Then on the Order Management tab is under purchasing. Allow Expenses on Purchases.
But @KevinJ of Kansas is right. It will remove the ability to add expese accounts to purchase orders as well, so if you use that I wouldn't recommend turning this off. But if you always add Items to your purchase orders then it will be fine
Or you could just make an Item for every expense account you want to cut a PO for and it won't matter.