http://forums.d2jsp.org/topic.php?t=56819794&f=258&o=20Quote
The ability to block users from posting on a public guild chat has been added. Please note that you can only block users on publicly postable guild topics. Restrict your private ones by guild rank.
To use this feature, click the Manage Block List at the top middle of your guild topic (near the Track Topic), then add a user by their name on the following page. To remove blocked users, click the X in front of their name. Please note that you cannot block your own guild members (they will be added to the list, but still able to post).