The only way to disable autofill is site by site, and then only by removing the keychain entry.
Apple changed the behavior of Safari in macOS 10.14.4, and you may have noticed it and thought it was a bug. Now, if you have stored a password for a website, when you select a login entry to autofill, Safari 12.1 for macOS automatically submits the login. Previously, it would fill the fields and still require you to click a Login or Submit or other button to proceed.
I understand Apple’s logic in making this change, as it reduces friction and takes less time to log into a site, much like dropping text message login codes into an autofill field for macOS and iOS. Apple described this Safari change in 10.14.4’s release notes as “Streamlines website login when filling credentials with Password AutoFill.”
So far, there’s no way to turn this autofill option off nor to choose sites to exclude. Some Apple forum users have complained that on certain sites that require a CAPTCHA—one of those distorted letters or photo-identification challenges—to be filled in that only appears after they enter credentials, the new autosubmit tries to login in and fails. (I’ve noticed this on some sites with CAPTCHAs, but others work fine.)
Read more at MacWorld.com