# Way 1. mod_rewrite way.
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} ^Mozilla\/5.0.*Googlebot [NC]
RewriteRule .* - [F]

# Way 2. SetEnvIf way.
SetEnvIfNoCase user-agent "^Mozilla\/5.0.*Googlebot" ua-var=1
<Directory /path/to/documentroot>
  # Add/Remove options. Up to you to decide what to allow.
  Options SymLinksIfOwnerMatch Indexes
  # Add override options if you want to allow htaccess.
  AllowOverride None
  # The following lines determines access to the site.
  Order allow,deny
  Allow from all
  Deny from ua-var
</Directory>