Instructions
Turn on the PS3 console and the XMB will be displayed on your screen.
Go to the "Game" folder displayed on the XMB and select the option that reads "Game Data Utility" from the folder's drop-down menu. The thumbnails of all the games you have installed in your PS3 will be displayed here.
Scroll the list and select the game that you want to delete by pressing the button with the triangle on it. This will display several options.
Select the option that says "Delete," select "Yes" to confirm and the game will be uninstalled. The thumbnail to boot the game from your PS3's hard drive will no longer be displayed in the "Game" folder.