The Finger of God is a Keeper which, if in play in front of you, allows you to "smite (i.e. discard)" any Creeper, at the cost of returning Finger of God to your hand.
The question here is what "smite (i.e. discard)" can possibly mean. If you're technically-minded, "discarding" in Fluxx would seem to refer to the act of discarding a card from your hand. In this case, the use of Finger of God would be: when you draw a Creeper, instead of putting it directly into play, you have the alternative option of putting it into the discard pile (and presumably drawing another card as usual). Under this reading, it's very difficult to see how you could use the Finger on other players' cards.
Fluxx though isn't really a game that benefits from the players getting too technical about the rules, in my opinion. I'd suggest that "discarding a Creeper" should be interpreted as "move a Creeper card that's in play to the discard pile". This is much more satisfyingly SMITEY, after all. BOOM!
Can this divine smiteage be employed against the Creeper of another player? Well, as any God-fearing knight would be able to tell you, the good Lord our God Jehovah is omnipotent and as such can smite any evildoer He likes. If you want to argue that He is limited to targeting some Creepers and not others then ON YOUR OWN HEATHEN HEAD BE IT.