Two ways of thinking, and the public perception is just as important for Walsh as it is to other young players:
(1) Tell him to **** off, he had his chance to develop properly through our system, he didn't want to work for it so left for a quick buck and a start. Once you show that attitude, you've proven you're not the sort of player we want, it makes other young players think twice about the easy road, and they'll know that if they stay they'll be expected to develop how the club sees fit.
(2) Let him come back, but with certain restrictions (expectation to develop further in Q Cup if necessary, especially in defence). They learn that they do get another shot but only if they have learned their lesson.
Coupled with this though, there needs to be the showing of reward for any loyalty shown. As much as Glenn and Boyd played on far too long, they were looked after due to their efforts for us. Even if it's not "veteran gets rewarded when past his prime", there needs to be mid-range and 4-5 year players that are very visibly rewarded for putting in their efforts to show that they are appreciated and looked after if they put in the hard work every week and put the team first.