Displaying Error message in Standard ME21N Screen

Hi all.

I need help.

During PO Creation in ME21N T-code, when we enter the vendor value and if the vendor is flagged for deletion(LFB1-LOEVM = X) , I need to display a error message that 'The Vendor &1 is flagged for deletion' if the user presses enter.

How to do this?

BADI's or Custom Exits???

Suggestions Please......