to Block a user from posting on Threads you start, on your Profile Page and from sending you Private Messages, you can use the Forum BLOCK feature.

you have to accurately enter the user's name in the "Block User" window and Save... this window is found under the "Customize Profile" tab, then the "Edit Ignore" list, which are features of your Profile Page editor. type in the name in the little line window, and "SAVE CHANGES". the name will appear in a list above the line window when done.

this will not block them in CHAT. for that you need to be in chat with them, not necessarily in the same room, and click on their name and select BLOCK from the drop-down menu. this should remain over time, even if you leave the chats and return. they will not be able to Whisper to you or use the Private Chat feature with you.

i encourage you to report harassing users to Admin (see who's on duty at the very bottom of the Community Forum), or a Moderator. if they are doing this to you, it is likely they will do it to another.

best wishes~
Gus