If they're harassing you on Steam (I don't understand really how this could be happening), but you could go onto their profile and select "More" > "Block All Communication"... this will prevent them looking at your profile/messaging you (I believe). Furthermore you could select "Report Violation" and choose "Harassment", do this to him and anyone else who you believe is harassing you (remember, there is a difference between him simply annoying you/trash talking you and harassment).