Have you ever had a guy turn nasty after you turn him down?

There is this guy at work. I tried to be friends with everyone when I started, including him and he made it clear from the start he fancied me. I made it clear from the start that I wasn't interested and he seemed to get the message and then about two months ago he sent me some suggestive texts about my body. Needless to say I didn't reply. Then I mentioned to a friend that I was seeing someone. Out of the blue the work guy asked me how things are between me and "my guy". Then he said "Is it an open relationship? Are you free to see other people?" I have since avoided this guy, who has started being quite nasty. He uses every opportunity to listen to my conversations with collegues and will jump in with comments like "I thought you were to posh for that place/that programme" and bizarre coments about my hair or makeup or clothing, which he will shout across the office. Has this ever happened to you? How do I make it stop? I dread going into work or even speaking and he has turned some of the other girls in the office against me by telling them I am "victimising him"? Help me please!