Very effective

I’ve run WordPress sites in the past where the comments would get inundated with spam within days of launch. The only thing that stopped it was adding captcha. But if you want to avoid having to force captcha on people (even google re-captcha is annoying with its stupid photo matching question. Right, “click all the pictures with steak in them”, and if your eyesight is relatively poor, you are shut out) CleanTalk so far is batting 1000. Just make sure you disable your captcha type plugins first.