Here is a list of IPs I have blocked associated with spam on my relay. Since I haven't solved the root of the problem yet, I've just been playing a cat and mouse game.
54.82.2.237
3.90.13.144
18.215.247.200
52.87.211.14
Discussion
Dynamic IP's are fun to block. Did you noticed thereply guy in this thread?
Maybe temporarily block all AWS: https://docs.aws.amazon.com/vpc/latest/userguide/aws-ip-ranges.html
You may be blocking some good relays, but not all of them. And certainly not clients.
You could then whitelist specific relays. Eh?
The best way to do this is to return a completely fake success response to the clients of these IPs, which can be delayed much longer than an outright ban. Alternatively, you can redirect requests from these IPs to a separate relay backend and continue to observe their behavior.