An alternative reply, with analogy, if you like them:

You own a restaurant, where you sell poisoned (intentionally and knowingly) food. A group of people band up for class action lawsuit for poisoning them, and have the lawyers post a sign at your restaurant, that everyone poisoned there should reach out and get some compensation.

Should you be allowed to take the sign down?

They shouldn't be allowed to put the sign up unless it's court ordered.

I know this answer doesn't pass the vibes test, but it's how the law actually works. If you post a sign on someone's property without permission, you'll get in trouble for trespassing, vandalism, or both.

So get a judge to issue an order. In a serious situation, they very well might.