Hunter indexes publicly available email addresses, which is its entire business model.
These guys ask you to fill in a fairly pointless form. Don't worry about it, just fill it in with junk apart from the email address you want removed, and hit submit. They'll email you pretty quickly with a confirmation link. Hit that and you should land on a "success" page.
Frequently asked questions
Will my data come back?
Possibly. Data brokers re-scrape public sources periodically, so your details may reappear after some time. It is worth checking back every few months and resubmitting a removal request if needed.
How long does this take to process?
Hunter sends a confirmation email very quickly. Once you click the link, removal is usually near-instant.