Hi, how do I unfollow someone? Thanks in advance.
Basically what 123cereza456 says. The button will then change to "Follow", giving you the option again to follow that user again.
If you want a user to stop posting on your stream or whatsoever, you can block the user by clicking on the "gear" icon on the upper right corner of their profile (it will say "Block [username]" and you will not be able to post on their stream or follow the user (unless you unblock them, which can be done by clicking on the same gear button and it will say "Unblock [username]":
Hi Syttamore and thank you for your answer. Actually, you're the reason I wanted to know. Not personal, actually the opposite. You're so helpful and so prolific on duo that I couldn't keep up with all your updates. Good luck with all your language learning!