Same here, I have no idea. This is what one of my Những người bạn on fanpop said.
Go to your messages and tiếp theo to the inbox/sent/trash options there is a "block users' option. Just click on that and add the usernames of the users bạn want blocked. Any further PMs sent bởi blocked users will automatically be discarded into your trash box. Hope this helps!
Didn't really work for me, but I guess bạn might figure it out, since I have a feeling bạn are smarter than me.