Why not do this in Apache? Use mod_rewrite, and you won't need to spawn new processes.

RewriteEngine On
RewriteCond  %{HTTP_USER_AGENT}  ^BadBot.*
RewriteRule  .* - [F]
[download]

Another solution could be to use a rewrite map that contains all the bad bots you know:

RewriteMap robotmap dbm:/path/to/file/map.db

RewriteCond %{HTTP_USER_AGENT} !=""
RewriteCond ${robotmap:%{HTTP_USER_AGENT}|NOT-FOUND}  != NOT-FOUND
RewriteRule .* - [F]
[download]

This does require the use of a dbm map, since the USER_AGENT can contain spaces. It also requires exact matches, so you need to keep your map up to date. It should be very fast though.

See also mod_rewrite manual.