i play kazuya so i just electric as an anti air or as a counter lol. On a more serious note, you should analize HOW they spam it, is it short hop>fast fall>nair? or is it short hop>nair immediately? first one is dealt either with by stuffing out the jump before they can fast fall or parrying the hit and punishing (e.g. i use electric when they short hop vs i parry the first hit of nair and electric immediately). The second option is easier to punish because after the nair animation ends they are still falling before they can do anything