The Trouble with Tor
In addition to CAPTCHAs, why not just have a button that runs some JavaScript that completes a proof-of-work similar to what mining bitcoins does? You could make it only take 5 seconds on a modern laptop CPU, about as long as it'd take to enter the CAPTCHA anyway, but it'd potentially be a very large road block for spammers/DDOSers.For those on phones, you can still opt for CAPTCHA if you don't want to kill your battery.