When jabber is disabled in board settings and you click on the jabber button next to the post (prosilver) you'll get a PopUp with this text:

Sorry, direct messaging of Jabber users is not supported on this server. You will need a Jabber client installed on your system to contact the recipient above.

In this case server settings of the jabber server don't matter, because the jabber feature is completely disabled in the board itself.

I would try making this a bit clearer in the text. Maybe just replace "server" with "board".