Create a custom vhost.conf file and populate it with the appropriate content, for example:
<Location />
deny from all
</Location>
Depending on your version of Plesk, this file will be created at the following location:
Plesk <= v11.0 : /var/www/vhosts/domainname.com/conf/vhost.conf
Plesk >= v11.5 : /var/www/vhosts/system/domainname.com/conf/vhost.conf
Then instruct Plesk to activate the custom vhost.conf via the following command:
/usr/local/psa/admin/sbin/httpdmng --reconfigure-domain domainname.com