After the PSN maintainance on the 17/11/2011, there is now a way to deactivate ALL ps3 and psp systems via the internet.
Note that you can only deactivate all via the internet once every *6 months* via pc so be careful.
To do this,
This will take you to the Sony Entertainment Network Account Page,
Log in and click on account at the top ( page view shown below )
Now click on Devices and media on the Left and then click on the down arrow on game and click deactivate all.
Read the message carefully, ( You can only deactivate all devices ONCE
every 6 months but you can go on your ps3 and activate/deactive unlimited times) and
Click OK.
***All PS3 and PSP devices for Game are now deactivated***
Start your PS3 up and go to Account Management and click on Activation, then Game and active your PS3.
Now you can all enjoy your old account and current accounts that were fully activated!
ENJOY!!!!!
Credits: Official Sony Websites.