I'd say scamming someone who has scammed you is morally incorrect, however, scamming them for something they scammed FROM you is not morally incorrect. This being that you are simply retrieving property that was originally taken from you.
Given that you would not be in that position had that person not done said scamming, and can therefore justify scamming back your property.
Let's take this a step further and say that killing someone for killing someone you knew/loved is unjust because that person who was killed will not get their life back simply because you killed the perpetrator.
In the event that you are scammed, and do decide to scam back what was taken, as long as no additional items are taken in the sense of 'retribution', then there is nothing morally invalid about what you have done.
At least, that's my opinion.