HTML basic form - needs help to block spammers

I have a Legacy Basic HTML form to process. It gets some spam and I would like to limit the spammers.

Right now I have a simple JPEG image with four numbers on it and one of the form inputs asks the user to enter the four numbers which is checked before processing. This old legacy System does not seem to work well anymore.

I still want the form to be fairly easy to fill out but I would like to improve the spam checking.

Question: can someone recommend some techniques that can be used to block spammers?

Thanks in advance - Dave

I’ve seen a display grid of images and the person is asked to choose the images that have something in them (like a bunch of city streets and select the images that have the crosswalk, or the images that have the bus). Computers have a hard time with this because they can’t process the images as quickly as humans. So you would need to find images that were obscure enough that a computer can’t process them quickly but clear enough for a human. They usually display a 3x3 grid and you have to find 3 images and then they do it again (another 3x3 grid) and that is usually all the checking done.
You’d have to research how much time a human needs vs a computer to provide the answers and you’d have to handle it when people are just failing and want a different set of images to review.

Not sure if any of this is available in a handy open source library though.

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Thanks for the idea.

Are you thinking of this: google recaptcha?