No way to stop it, all you can do is try to make your team seem like a good place to be.
Of course "good" means different things for different players.
Some players prize loyalty, other prize getting to play a certain style, others want to be empowered to do whatever they want on the floor, etc etc.
I would say that the strategy the Celts seem to have taken this summer, i.e. prioritizing adding a lot of good character guys to build out the back half of the roster, is a good idea. Especially since the Celts need to rehab their image after a year in which many of the players on the team were obviously unhappy.
Used to be that we'd hear all the time that players around the league really want to play for Brad and that everybody agrees that it's really great to get to play for Brad Stevens.
The Celtics need to get back to that.