Right click on POSTAL 3 on the sidebar of your steam library.
Click Manage > Uninstall.
A confirmation window will appear, select uninstall.

Click on POSTAL 3 in your steam library.
Click on the support button located along the bar of options / links for the game.
Select the option "I want to permanently remove this game from my steam account"
Select "Ok, remove the listed games from my account permanently"

You have successfully completed POSTAL 3!

Obtain gasoline.
Pour the gasoline all over the floor of your house.
Obtain a match or lighter.
Light your house on fire.

Optional: If you wish to remove the psychological damage of playing this game, stand inside your house as it burns.