Instructions
Turn on the PS3 system, and scroll over to the "Users" section of the XMB. Highlight the user account tied to the PSN account that you want to deactivate and press the "X" button to log into the account.
Scroll over to the "PlayStation Network" section of the XMB, and scroll up to the "Account Management" option. Press the "X" button to select the option.
Scroll down to the "System Activation" option, and press the "X" button. Highlight the "PS3 System" option, and press the "X" button.
Highlight the "Game" option, and press the "X" button.
Highlight the "Deactivate System" option, and press the "X" button to deactivate the system.