in the .htaccess file,
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_USER_AGENT} ^whateveragent1.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^whateveragent3.* [OR]
...
RewriteCond %{HTTP_USER_AGENT} ^whateveragentn.*
RewriteRule ^/index.php http://example.mobi/otherurl.php [R=301,L]
That should do it.
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_USER_AGENT} ^whateveragent1.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^whateveragent3.* [OR]
...
RewriteCond %{HTTP_USER_AGENT} ^whateveragentn.*
RewriteRule ^/index.php http://example.mobi/otherurl.php [R=301,L]
That should do it.