We haven't seen webmention spam in the wild yet (except perhaps, as Barnaby has pointed out, Twitter spam that's been imported). But we do have a solution.
You'll find that Known becomes more and more social over the coming months. Your social graph - the list of people you're connected to, and they're connected to, and so on - can be an important tool for all kinds of functionality. Some relate to content discovery and community, but one of those things is definitely spam prevention.
Unlike, eg, WordPress, Known has been built in the era of pervasive social networks, and we embrace this wherever possible. One way to tell if something's likely to be spam is to see who it came from, who they're connected to, and if they've been previously flagged for sending spam. We intend to do this, in addition to the flat content filtering that traditional spam services do.
Worth reiterating here: Known is different because you have full control. We're never going to mine your data, and you get to pick which metrics your site uses.