How to prevent user from spamming my newsletter subscription?

How can I prevent users from just subscribing, and then refreshing the page to keep doing it over and over again? I have tried to put a recaptcha in there but its not working for some reason...Any insight on to why that might be? Also are there any other steps I can take to prevent this?

      <form action="insert.php" method="post">
        <div class="form-row">
          <div class="col-12 col-md-9 mb-2 mb-md-0">
            <div class="g-reCAPTCHA" data-sitekey="6LdlkjsdflAP-lBcFklj3242jlkG3sm-rr1"></div>
            <input type="text" class="form-control form-control-lg" placeholder="Enter your email to subscribe..." name="email">
          </div>
          <div class="col-12 col-md-3">
            <button type="submit" class="btn btn-block btn-lg btn-primary">Sign up!</button>
          </div>
        </div>
      </form>