You can also be sued for defaming someone, but the point of free speech is except in very rare circumstances, the government cannot imprison you for your speech. No one is forced to provide you with a platform to exercise that speech, so you are right that forums can ban you if they disagree with you. Newspapers and magazines don't have to publish our opinions either. But even heavily moderated boards do not treat criticism of the FO as rule infractions subject to banning.