I'm in a healthy relationship of almost three years with someone, but he was in a toxic relationship with an ex who asked we never talk to her again. I feel like we can't engage in the poly community because she's talked shit about my partner. She's made so much noise that people are telling my friends to stop associating with us, even though all of the things they "know" about us are from her perspective. She's burning our bridges before we even know they exist. Is there even anything I can do?
Unfortunately, this happens - not just within polyamorous communities, but also in queer communities and kink scenes and plenty of other types of communities. I've seen this happen before, to myself and to people I'm close to. The best thing to do is to keep your chin up and your head down. Don't engage with the gossip or do anything that would prove this person "right." Be responsible and healthy and genuine, and the people who can see clearly will be able to see the truth. In the end, the person spreading lies and starting drama will not come out on top.
You may want to be a tiny bit proactive - when meeting a new person, say "I just want to mention that there are some rumors going around about me, so if you hear something that concerns you, I'd be happy to talk about it and clear things up." Be judicious about this, though - it could backfire and just send a red flag that you're a source of drama and gossip. So save it for when you really do think this person will hear the rumors and seems like the type to take you at your word and bring it up for honest communication.
Your best bet though, as frustrating as it will be, is to just wait this out. People come and go through communities, rumors burn hot and then burn out, and if you're doing the right thing, you'll come out on top. You'll probably miss out on a few potential friendships and relationships because of poorly timed gossip, but that's unavoidable even if there isn't someone out to poison the well against you.