OK, try this...............make a duplicate of what you have so we don't mess it up.
On the duplicate, Go to select, color range, set the fuzziness to 70, with your mouse select the smoke coming out of the gun, click OK.
You should have a marching ant selection of the smoke and the "Dirty Harry" text.
Fill the selection with white. (Paint bucket tool)
Hit Command/Control + D to deselect.