UPDATE:

This extension is no longer maintained.
Because of latest changes to Slack's frontend, the message sender's details are not attached to every message. Which means, to check if a certain message was sent by a particular sender, we will have to backtrack till a message where the sender's information was available. Doing this for every message will be inefficient and will lead to unnecessary CPU usage for the User.
Therefore, I will be archiving the extension now 😢

Hide messages from specific users, make Slack a Happy Place Again! 😁

Install:

Install Happy Slack! from Chrome Web Store.

Features:

1. Hide messages from any User
2. Hide messages from any bot
3. Optionally allow messages to be hidden only from DMs

How To Use:

1. Click on the extension icon or navigate to extension options.
2. Enter the ID of the user/bot you want to block. NOTE: This ID is not the slack handle the user/bot has, but the unique ID that slack assigns to every User/Bot.
3. You can get the ID by:
   1. Click on the user's/bot's name/photo from any channel
   2. Select View Profile
   3. The user/bot's profile will open in the side bar on the right hand side. Click on the downwards arrow icon besides Message.
   4. Select Copy Member ID
4. You can block multiple users or bots by specifying their User IDs separated by ,.
5. Make sure you want to make Slack a happy place again.
6. Save!

Warning (from Slack, although the developer here does not, in any way, even accesses your data)

This extension is not officially supported by Slack and is not listed in the Slack App Directory. Your data — including messages, files, and profile information for all workspace members — may be exposed to the developer, so understand the risks before installation.