Probably not, no.
Metamagic feats frequently say “you can” do something special with a spell, whether that’s change its type of energy damage, change its range, or increase its area. Saying “you can” typically means “you don’t have to,” however in the case of metamagic feats, the choice whether or not to do so happens when you choose to use the metamagic effect on your spell or not. So if you prepare spells, and you prepare it with metamagic, that thing you “could” do is now something you have done, and you can’t undo it.
However, nothing in the rules outright says that. I have always played that way, no one has ever questioned it when we played that way, and every discussion of metamagic feats I have seen has also assumed that once applied, metamagic effects are not optional. Some metamagic feats even seem to rely on it for balance, since they change the spell in a way that might be good, or might not, and so do not require (as much) spell level adjustment. Energy Substitution, for a prime example. Furthermore, the ability to spontaneously use metamagic is supposed to be why spontaneous spellcasters take longer to use it, which gets eroded if wizards can just opt-out of metamagic (though, of course, the wizard has still paid for it in spell slot level).
And if you use Smiting Spell, that is, you actually put the spell effect into the weapon, you definitely can no longer just touch someone with it: Smiting Spell “alter[s]” the spell so that it discharges on “strik[ing] an opponent with that weapon,” so just touching them isn’t going to work anymore.
So you can ask your DM, but I suspect the answer would be “no.”