Not sure if I got this right: You are trying to remove PRODUCT Tags for a specific SUPPLIER_PID? If so try this:
Go to Search->Replace
Search for <PRODUCT mode="new">\R<SUPPLIER_PID>285129</SUPPLIER_PID>.*?</PRODUCT>
Replace with nothing
Select Regular Expressions. Make sure “. matches \r and \n”
is checked
Hit “Replace all”
But if you have to do this kind of job on a regular basis, you may want to look for a Tool that is more specifically made for manipulation of XML by XPath.