Instructions
Turn on the PS3 system and log in as the user that the PSN account is associated with.
Scroll to the "PlayStation Network" section of the XrossMediaBar and select the "Account Management" option.
Select the "System Activation" option, followed by the "PS3 System" option.
Select the "Game" option, followed by the "Deactivate System" option. Select the "Video" option, followed by the "Deactivate System" option.
Scroll to the "Users" section of the XMB. Highlight the user account, press the "Triangle" button and select the "Delete" option. Confirm that you want to delete the account in order to remove it from the system.
Repeat these steps for all other PS3 systems associated with the PSN account.