Coaches are, but the student athletes who move on would not be restrained. Most coaches move "UP", and will be focusing on players who are at the higher level. A coach taking a job at a major conference school is not going to be around very long if he takes the 6'8" center from the small mid-major. That coach had better be focusing on higher level players to compete night in and night out with the higher level game.
Student athletes may not be restrained,but you can restrict a coach's ability to take players to another program.
I would think you would include language in their contract similar to a buy out clause.
As far as a coach taking players with him.
It depends on who the coach has playing for him.
Are you suggesting that if a coach has a Gary Trent like player,he wouldn't want him to go with him to another program or that that player wouldn't be capable of playing at a higher level ?