@emily crypto id maybe? so you can keep a list of abusive identities and share a reputation graph?
this is a tough one tho, esp when identity isn't persistent. alternatively, a whitelist with some vetting process.
http://discolab.rutgers.edu/byzantine/publications/byzauth05.pdf
expresses one possible p2p auth system
the #crypto part is so that you don't have to manage transmitting trust manually, you just keep track of who you trust and don't.
you need WoT to boostrap this tho.