From the version of 6.2.6.3 we have given a new option of Don't Allow to edit rates with Purchase from PO only. When enabled, the rate editing will be restricted when a purchase order is loaded in purchase screen. In order to enable this option please do follow the below steps:
Step1: Go to Tools->Options->Options->Purchase->Purchase settings->Purchase from Purchase order only and then enable the Don't Allow to edit rates with Purchase from PO only.
After enabling the above mentioned option,when you try to change the rate in the purchase entry screen, there will be message poped up as shown in the image below.