How could someone give bad review wrongfully and still asking for stay ?

I got one guest for 1 night and next day he asked to stay longer but I couldn't give him the room as I had previous booking. Then he left but continuously texting me for more days when I told I have full booking then he posted negative review which was totally lie and without any proof. Previously 1 guest took my key and never gave it back even I requested him to post it. What should we do to prevent these kind of incidents as booking.com is not very much helpful for host's interest?