People are allowed to disagree. They are allowed to use the F bomb. They are allowed to contradict your posts with posts of their own, even numerous ones.

What they are NOT allowed to do is make threats of violence against you, such that the threats would be seen as credible and create fear to a reasonable person. If you've got threats against you, you file a report and the police can determine if the threats indeed constitute a harassment or assault crime. If they determine that it doesn't (and unless they made very CLEAR AND SPECIFIC threats, it won't), your only rescurces are to ask the moderator to intervene (which you can't FORCE), to get thicker skin, or abandon the board.