Sometimes the scar tissue may itch, and of course once they start licking, it goes on and on. I would try something like tea tree oil, not on any irritated skin, but near it. My Topper would not put his face anywhere near that scent, and it would keep him from gnawing his tail.
++ on the tea tree oil - I've used it when Gemma gnaws on her knees (who knows why, she has issues), and even on furniture when Shaye was in her teething stage - they really don't like it at all:(