Vikas, if I'm understanding your description correctly, you have a redacted, purchased mailing list (lacking email addresses) against which you want to match form fillouts. I'm assuming everyone on the list is a Marketo lead.
First, the only way a form fillout is going to be matched with a lead based on FN/LN/Company is if you talk to Marketo support about changing the primary key in your Marketo instance from Email to the composite FN/LN/Company. This is not something you can do on your own.
Second, you're assuming people are going to enter their FN/LN/Company exactly as it already appears in your lead database. This, I can guarantee, is not going to happen. It may happen in many or even most cases, but far from all. Therefore you'd need to pair this approach with a data cleansing service such as those offered by RingLead. Being unable to use the email address as part of deduping is kind of ridiculous, but you may still get positive results.
Third, and to your direct question, you cannot stop someone from filling out the form if someone forwards them the link. The form is going to look the same for them as for the original recipient and doing real-time fuzzy matching before form submit isn't feasible (unless you happen to have a very capable developer). That's actually not a big deal in itself, as you can let the person submit the form and just delete the lead from your db immediately if the back-end match detection (including RingLead or similar webhook) doesn't come up positive.
I should also add that the way the list provider is handling this send is well nigh insane. A professional operation would, at the very least, tag each link with a locally unique identifier that would become a hidden field in your form, and which you could in turn use to find leads in your db, with no mistakes.
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