I've decided to return to my Taking Out The Trash series after a long break. The Smith Morra gambit is without doubt the most respectable gambit that I will try to 'refute' in this series. I recommend you use this particular line only as a start of your investigation.
One last disclaimer: 3. ... Nf6 is an alternative to accepting the pawn sacrifice. Black may force the game into a Alapin (2. c3) Sicilian this way and, as long as he knows the theory, will get a perfectly fine game like this.
Personally... I grab the pawn.