Stop multiple form submissions from same user
I swear I've seen a post on this before, but after a few hours of trying to search for it again I'm just going to ask myself (I'm sorry mods). I've been trying this for quite a while now and think my brain just isn't working correctly anymore so any help is appreciated!
Context:
We use a Marketo form to 'pre-qualify' potential customers before they're able to create an account. Every user that makes an account, our sales team reaches out to and they spend 75% of their disqualifying people. Since we're a start-up, our resourced are limited. It has very simple logic where if you select x, you are redirected to a page that says you don't qualify. Needless to say, it didn't take people long to figure out if they change their answers it will show they qualify and they can create an account.
I'd like to be able to stop people from just hitting back and resubmitting it with changed answers. This form is embedded on an external page which complicates things. I'm probably completely missing the mark, so I'm completely open to suggest on how best to go about this!
I've tried a few things, but here's my latest setup:
1. I created a checkbox called getStartedFormFilled and it is a hidden field on the form

2. I tried to write some script that checks the box on submit. I consolidated it with other script I have but I'm very rusty on my coding:
<script>// <![CDATA[
MktoForms2.whenReady(function(form){
form.onSubmit(function(form){
// check off the checkbox
var checkboxField = "getStartedFormFilled";
if (form.getFormElem().find('[name="' + checkboxField + '"]').length > 0) {
form.getFormElem().find('[name="' + checkboxField + '"]').prop('checked', true);
}
// consolidate fields
var currentValues = form.getValues(),
fieldsToConsolidate = ["Where_is_your_business_registered__c","Do_you_invoice_businesses__c","What_Industry_is_your_business_in__c","How_much_funding_are_you_looking_for__c"],
getStartedConsolidated =
fieldsToConsolidate
.map(function(field){
return currentValues[field];
})
.join(";");
form.addHiddenFields({ getStartedConsolidated : getStartedConsolidated });
});
});
// ]]></script>
3. Then I added a redirect choice where if this is checked, to go to a different page. I have 6 other options that disqualifies them based on their answers and they all redirect to a different page. Otherwise it goes to a qualifying page. I made this choice 1 so it supersedes the qualification.

What am I doing wrong?