I know some gentiles who ask "What can I do about antisemitism?" and the answer here is simple:
1. Stand up to it. Make comments on such most calling it out. Make the person who posted it have to deal with being called out.
2. Report the poster to your local admin. And follow up. If they don't take action, ask why not and see if you can get them to change their mind.
3. Repost these post. Get the word out.
4. Reach out to your Jewish friends. Reach out and ask "Are you okay?", check in to see how we are and offer your support.
When you do this, we feel seen and cared for. When you don't ever stand up for us, we notice. We might not tell you, but we do.