First locate the virtual host configuration file for Apache related to the domain, on FreeBSD it should be here:
cd /usr/local/etc/apache/vh
Now add following in between the begin <VirtualHost *> and end </VirtualHost>
RewriteEngine on
RewriteCond %{HTTP_HOST} ^www\.(.*)
RewriteRule ^/(.*)$ http://klauskjeldsen.dk/$1 [R=301]