You can block messages from a UIN (User Identification Number);
If you wish to block messages from a particular sender, you can use this feature.
To Block a particular user, right click in the Online Contact list to bring up the menu. Then, Select Blocked UINs. This will bring up a small window. Within this window will be a list of UINs to block.
To add a UIN to the Block List, right click in the window and select "Add UIN".
To delete a UIN form the Block List, click on the UIN to higlight it. Right click in the window and select "Delete UIN".