In Firefox
Go to Tools >> Add-Ons >> Preferences (next to Adblock Plus) >> Filter Preferences.
Select the filterlist you want to delete, click on the "Action" button, and select "Delete".
Click on "OK" to confirm.
In Google Chrome
Go to Window >> Extensions >> Options (of the Adblock Plus extension).
Click on the red "X" next to the filterlist you want to delete, and click "OK" to confirm.
In Opera (on Windows/Ubuntu)
Click on the menu button and go to Extensions >> Manage Extensions >> Click on the tool-icon >> Preferences.
Click on the red "X" next to the filterlist you want to delete, and click "OK" to confirm.
In Opera (on Mac OSX)
Go to Tools >> Extensions >> Manage Extensions >> Click on the tool-icon >> Preferences.
Click on the red "X" next to the filterlist you want to delete, and click "OK" to confirm.