One way to make your Joomla forms more secure is to make users subscribing for the first time double opt-in before you add them to your email list. With double opt-in, a user will submit your form and then receive an email asking for them to verify their email. Usually, this includes a verification link that the user will click on.
Convert Forms is not a newsletter system and as such it doesn't have double opt-in functionality available. However, all of the email providers that integrate with Convert Forms allow you to enable double opt-in easily:
If your email service provider isn't on this list, that's okay! With our Zapier addon, you can create automation that trigger confirmation emails with your favorite email provider.