I had read (in a comment here, so take with a grain of salt) that some had started doing Proof of Work.
I.E. they ask the visiting computer to do some math. This is potentially less annoying to people than clicking on traffic lights or typing unreadable text, but could get costly if you're using bots.
Settings -> Comment Customization -> scroll to the bottom
I don't know if that does exactly what you're asking for, I just turned it on myself