Free speech means the government can’t stop you from saying something. It does not mean you have the right to be heard, or even to force people to listen, or that private companies (like Lemmy) can’t remove your ability to post to their platform. Hell, it doesn’t even mean you can say whatever you want without consequences, which can range from being blocked to being banned all the way up to being arrested if it’s a threat.
So, feel free to say whatever you want, just don’t be surprised when you lose your ability to post.