You can block someone from their profile or from a chat, for any reason, at any time.
Once you block someone: they disappear from Discover, your matches, your chat list and the Ports tab, and you disappear from theirs. Any match you had with them closes right away, and neither of you can message the other again.
The person you block is never told that you blocked them.
You can undo your own block later, from “Blocked users” in Settings. Undoing a block does not restore a closed match or old messages.