Nothing at the protocol layer. Just handled by relays, users, and clients with varied approaches. I suspect the best long term approach will be web-of-trust related. But you can report them to relays and they will ban or limit certain types of access. Some relays share how they treat things like that or specifically use WoT as their filter and you can add those relays, etc.

Bunch of different options, probably standards will emerge with enough time and after we learn from a few really annoying bot attacks. Only had one or two significant ones so far.