[Egrodo]

If it becomes this easy to block traffic couldn't malicious applications really mess up a user by spamming out reject packets for common IP?

[Ajedi32]

I think it would have to be something like "Block traffic from <offending IP> intended for <my IP>. <TTL>. <Cryptographic signature verifying that I control my IP>."

[codedokode]

The intermediate routers can send back a confirmation code, and you must send a new reject packet with this code to confirm the ban.