Accepted a friend request from someone after playing against them, and they started to harass me.
We recommend you don't accept friend requests from strangers.
If someone on your friends list uses inappropriate language or leaves unwanted comments, open your friend list in the Battle.net desktop app, then right-click their username and select Remove Friend. If you want to report them for harassment or for an inappropriate name, select Report from the same right-click menu.
You can also right-click their portrait in a chat and select Block to prevent further communications.