I don't know about football, but I know some businesses have "no competition" clauses in their contracts. For a set amount of time (one or two years) a person can't work for a competing company after leaving the initial company. I don't know if that's legal in football or not, but those kinds of clauses do exist.
In the real world, covenant not to compete clauses are not always enforceable. I have not signed one but I have had clients who beat them in court.
The courts have deemed that a person has a right to earn a living.
The clause has to be limited. A certain length of time. A certain territory limit (Usually no more than 50 mile radius). And the person who signs the clause has to be compensated (more than just the job itself) for giving up his rights. $10,000 was the value most often used 10-15 years ago.
As for the NFL, I do not know what the compensation would be if GB would require the new team not trade Favre to certain teams but I don't see why they could not reach some kind of agreement on the issue.
