Since your effectively still allowing Community Users to submit cases (via your custom bug reporting tool), you can't actually strictly prevent them from filing cases via the web UI.
You can, however, use a BugMonkey customization like the following to point them in the right direction:
name: Prevent anonymous/community users from filing cases via the web UI
description: Hides/redirects the public case submission page to point to our custom bug reporting tool
author: Dane Bertram
version: 1.0.0.0
minApi: 1.0
js:
var sReportingAppUrl = 'http://your_custom_case_submission_app.com';
// if you'd like the "Enter a New Case" link on the login/community user homepage to *redirect* use this line:
$('a[href="default.asp?pg=pgPublicEdit"]').attr('href', sReportingAppUrl);
// if you'd just like to remove the "Enter a New Case" link altogether, uncomment the following line:
// $('a[href="default.asp?pg=pgPublicEdit"]').parent('li').remove();
// if you'd like to also remove the "New Case" navigation menu link, uncomment the following line
//$('a[href="default.asp?pg=pgPublicEdit"]').remove();
// if they've somehow made it to the anonymous/community user case submission page redirect them
if(window.location.href.indexOf('pg=pgPublicEdit') > 0){
window.location.href = sReportingAppUrl;
}
css: