http://store.playstation.com/login.gvm>>> Note that you can only Deactivate All via PC once every 6 months so be careful <<<
To do this...
Go to:
http://store.playstation.com/login.gvm Click on "Manage Your Account"
This will take you to the Sony Entertainment Network Account Page.
Log in with your PSN login and click on the "Account" tab at the top.
Now click on the "Media and Devices" option on the left side of the window.
In the square that says "Games", click on the downward arrow below (or anywhere in the square) where it lists how many PlayStation systems you have activated.
Now click on the button that says "Deactivate All".
Read the message carefully and click "OK".
All PS3 and PSP devices are now deactivated.