There are some advanced settings, that are only accessible by editing them in the config.xml file. (found at %appdata%/Bertware/BukkitGUI/Config or [your_server_folder]/BukkitGUI/Config)
Where to find them
In config.xml, search for <advanced> and </advanced> All advanced settings are between those 2 tags
Making wrong changes in config.xml can break the GUI. In this case, you either have to fix the .xml file or delete it (it'll be regenerated). If you delete it, all settings will be lost. Making a backup of hte file before continuing is recommended.
Between the advanced tags, you'll find <spamfilter></spamfilter>. Between <spamfilter> and </spamfilter>, you can write the text that should be filtered. e.g. write warning, and all text containing warning will be hidden. You can add multiple <spamfilter></spamfilter> lines , e.g.
<advanced> <spamfilter>hide</spamfilter> <spamfilter>this</spamfilter> </advanced>
all text containing hide or this will be hidden.
If you want to raise the quality of your avatar images, you can edit their size. If you have a lot of players,and you want to speed up the avatar loading, you can lower this value.
The value is in pixels (heigt*width, default is 32x32, so just 32 in the .xml)