Instructions
Press the "PS" button in the middle of your PS3 controller.
Select the "Turn off System" option, then select the "OK" option to turn the PS3 off.
Turn the PS3 back on by pressing the "PS" button again on your controller.
Select the "Game" option in your PS3 main menu, then select the "Grand Theft Auto IV" option. You will now be able to play the game with all trophies unblocked. However, unblocking the game will block the trophies again.