From Dynamics GP Essentials: GP 2015 – Warning of Vendors that have Purchase Orders. This is one of the features that I asked and voted for via MSConnect and now we have it in GP 2015.
Dynamics GP Essentials: GP 2015 – Warning of Vendors that have Purchase Orders
Payables & ReceivablesSales, Purchasing & InventoryGP 2015
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.