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The problem here is it's difficult to implement, you can blacklist domains but there's so many URL forwarders out there you'll never be able to block them all. It's also trivial to create your own using free services such as Github pages. One solution could be to check URLs when they are submitted, see if they lead to a redirect as that may indicate a shortener service, though there's so many legitimate uses that this too falls short. Hubski doesn't even use captchas at sign up, that small measure could help reduce some of the bulk automated spamming.