As Rafael points out in the comments, the pattern nēmō nisi is attested Classically, though personally I would use nisi quam amō or the like rather than nisi amor. To use Rafael's example, from Cicero's In Pisonem 7:
…ut nemo, nisi qui mecum esset, civium esse in numero videretur.
…that nobody seemed to be among the citizens, unless they were with me.
Nisi literally means "unless", so it needs to be followed by a full verbal phrase. Your nisi amor is perfectly valid, as a shorthand for nisi quī est amor, but the full phrase sounds better to me.
If you want a word for "except" which is followed by a noun, I would use praeter. The "except" meaning is more common in Later Latin, but also shows up in Classical times. For instance, from Cicero's Letters to his Brother Quintus 1.1.5:
…etiam e Graecis ipsis diligenter cavendae sunt quaedam familiaritates, praeter hominum perpaucorum…
And even from the Greeks themselves, certain intimacies must be carefully guarded against, except those of a very few people…
Interestingly, praeter with this meaning can be either an adverb or a preposition taking the accusative. So for your sentence, I would say:
Nēmō praeter Amorem mē impūnē lacessit.
Nobody except Love provokes me with impunity.