Hot -
First I would talk to the guys over at cgi works and see if they can't add IP blocking.
Does the submit software give you the ip number of the submitter when they sign up ??
If so you can use .htaccess to block that ip from even being able to see your site. The thing to do is set your submit area behind a password protected area. Then post that username / password to let anyone in and submit. You can then go through and use this in your .htaccess to block IP's from getting in. Notice the first one is a single ip #, the second one is a full class C which should catch anyone that is coming in on a dialup connection
AuthType Basic
AuthName Members
AuthUserFile "/path/.htpasswd."
AuthGroupFile /dev/null
<Limit GET POST>
require valid-user
deny from 200.31.48.11
deny from 215.2.4.
</Limit>
Tim