Have you tried fail2ban[0]? It can take log output from Asterisk and automatically insert iptables DROP rules for the source IP to block the traffic in the kernel. It still shows up on your interface and uses your bandwidth but dropping the packet in the kernel is much more efficient than Asterisk dealing with it (not to mention safer). It should also cause the bad actor to eventually give up on you and move elsewhere.
[0] - https://github.com/fail2ban/fail2ban/