htaccess File
www.seoipb.com/forum/index.php?showtopic=2834
www.seoipb.com/forum/index.php?showtopic=2834&st=0
Both of the above examples produce exactly the same content. However, in this case we chose to use htaccess to do a 301 permanent redirect from the second to the first. With this method, you shouldn't see any loss of traffic and search engines are likely to quickly update their cache to the URL you favour.
Add the following (change the parts to make it work for your own site of course) lines to your htaccess file. This can be in your domain root, or your forum directory...
RewriteCond %{QUERY_STRING} ^showtopic=([^&]+)&st=0$
RewriteRule ^forum/index\.php$ http://www.seoipb.com/forum/index.php?showtopic=%1 [R=301,L]