One way I like, as I dislike captcha images these days, is to add a hidden text box to your form, but don't apply any hidden attributes to it, rather set the background colour and borders to the background colour of the form, and set the font size to 0. That way a spam-bot will fill in the text box with its usual junk, but a user won't be able to see it to enter anything, so if the text-box has content in it, prevent them from proceeding.

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The man who comes back through the Door in the Wall will never be quite the same as the man who went out.