Hi,
It is very hard to stop somebody to order, it sound like you have freak after you. If you can check his/her IP and then you can deny access to your site from that certain IP from your .htaccess file
Put this code (IP is example ip) in .htaccess below RewriteEngine on
order allow,deny
deny from 123.45.6.7
deny from 012.34.5.
allow from all
If he/she uses different proxy servers and order then you have to put each IP in "deny from" list