You can look in your Seller Admin > View/Download Transaction Log to find the IP used for any transaction, but you will likely discover they were using a different IP for every checkout, since most computers get assigned a fresh IP from their ISP's common pool every time they establish a connection to the Internet, and most fraudulent buyers are careful about covering their tracks this way, or by using various public wifi hotspots.
For these reasons, and the high likelihood that blocking by IP will not effectively block a skilled fraudster but may instead wind up blocking legitimate buyers who happen to get assigned the same IP later from that same ISP's pool or happen to use the same public hotspot, we do not support blocking transactions by IP.