Step 1

- Click the "Help" tab at the top-left of the screen, and click "Steam Support" in the drop down menu.

Step 2

- In the search bar, type "DayZ".

Step 3

- Under "Products", click on Dayz.

Step 4

- Click "I want to permanently remove this game from my account".

Step 5
- Press "Ok, remove the listed games from my account permanently".

Step 6

- Never come back.