This script allows you to hide all the messages from a particular user on a particular Slack instance. The offending messages will be replaced by a red bar. You can hover over the "read" word in the bar to read the message.

All you need to setup this code is the id of the offending user, and the instance name (which is trivial, it's the name of your company, or your project...). To retrieve the user id you can right click on their message and check the corresponding HTML. You will find the id in the data-message-sender attribute of a element.