When you block a user on Discord, you still get an option to show the user's messages. This can defeat the purpose of blocking, it can be bad for your health.
The following CSS snippet will hide a blocked user's messages for good. It can be used with Stylish or any other extension that allows override a website's style.
[class^="groupStart"]:has([class^="blockedSystemMessage"]) {
display: none;
}