Spam, bots and scams aren't speech in any meaningful way. They don't attempt to say anything. Threats of violence and abuse may be speech in some sense but is in no way part of a dialogue.
Hate speech, scams and spam are all speech and they should all be removed and not tolerated.
To clarify, are you saying that when threats are censored it's not an encroachment of free speech because they aren't "part of a dialogue"? If so, then how is holocaust denial any different?