I had a guest book a whole month at my place and then started threatening and harassing me when I wouldn’t extend his reservation to 6 months . I offered him two and he became irate .
I called AirBnB and they basically told me nothing can be done . They obviously don’t care about the safety of their hosts . I am scared **bleep**less and lost out on my business which hurt me pretty bad and they simply don’t care
If the harassment is in the message system, then you can report such a message to Airbnb using the report option ("flag"), which also provides the option to block the guest from further communication with you.
And when a reservations ends, then just do not extend it if you do not want to.
Is the guest already at your property? If so, this should be a matter for local law enforcement. You can find more information here.
If they aren't at the property and the discussion is around a future booking I would recommend taking @Emiel1 's great advice and flagging the messages as abuse.