a lot of bots dont recognise robots.txt but follow some of the advice here
http://antezeta.com/news/avoid-search-engine-indexing. For .htaccess use this
Code:
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} (googlebot|bingbot|Baiduspider|yandex|anywordyoulike|like|bing) [NC]
RewriteRule .* - [R=403,L]
just keep adding names to the string or use my mod for banning bots.
Your .htaccess file must reside in the forum root for this (you might have to set your control panel to view hidden files if you dont see it)