How are you planing on identifying such a website visitor for already having filled out a form in future? You will either create duplicates over and over again, or, an this is more likely, no leads at all because blocking a text article - unless you mean to sell the information in it - is a bad practice. Also, consider SEO on your website. A bot will not be able to access the content either.

In any case, if you would want to limit access to a group the website visitor has to be a user in the first place. It's still a way to go from there, but this is the base. Else you would need to set up some sort of cookie management that tracks whether a user has filled up the form already, which is even further from the out-of-the box behavior of Odoo.

Alternatively you could create 0-eur service products per article and have the article be sent to the users upon completing the checkout as a PDF. This way they receive a document, so they don't need to rely on cookies to be known in future and you can be rather certain that you've received a valid mail since otherwise the user wouldn't receive the document (let's ignore throw-away accounts and 10-minute-mailboxes for the moment). This can be done with guests as well and doesn't necessarily require registered users.

Or, you could use contact forms with dedicated response mails - each again holding the article as a PDF.

In my opinion:

You should keep your blog post public and convert the website visitor due the content of the post by the means of a contact form at the bottom of each post. I usually wouldn't fill up forms prior to knowing what I'm getting from it beforehand.