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It takes some time before robot.txt changes reflect on the search results. Some engines behave also better than others.
Which engine are you talking about?
The example is for: XOOPS root = web root.
If you want to get rid of engines on the forum, place an .htaccess file in /modules/newbb/ and block their referer or IP range. Beware, some drastic measures can harm your sites' page rank.