The block feature can be added from the contacts list; you just scroll down, choose the person you don’t want to receive calls or messages from, and tap “block contact.”
I don’t yet see the block feature on the Android version of Alexa, but hopefully Amazon will update that app quickly so that everyone can have greater control over who has a quick line into their home. (You’re unable to block users from Alexa’s web settings, so the app is required.) Unfortunately, turning off calling/messaging functionality altogether is still too difficult for customers, as it requires a phone call to Amazon customer service. Ridiculous.
Read more from The Verge:
Watch: Free calls and messages on all Echo devices