Custom Form on Joomla site not protected

I have an HTML form on https://ironrosesister.com/blog-signup. It is a Joomla 3.10.11 site. I have installed the Cleantalk plugin and have checked all the checkboxes for “Forms Protections” including “Custom Contact Forms”. I have also disabled the Google Recaptcha plugin. (I would rather not upgrade this site to Joomla 4 if I can help it because there are so many different plugins that might break.)

Cleantalk is properly protecting the Joomla contact form, but not my custom form at https://ironrosesister.com/blog-signup. We get about 40 spam a day through that form.

The page I need help with: [log in to see the link]