I'm sorry to hear your having issues with a member of the site - have you reported the posts to the moderator of the section? Abuse is taken very seriously and it will be dealt with by the staff (you can either use the report function on the message or PM one of the section moderators).
You can block somebody - simply click on the user name you wish to block and view profile, you then have the option to 'ignore' that person. That means they cannot PM you and any posts they make is hidden from you so you don't have to bother reading them etc.
I hope that helps