From the cheap seats...

Just hand any out of position player an across-the-board 25% reduction in stats when they join the game.

To counter those who'd exploit switching positions to avoid the above, issue an opinion drop to the player. Any player whose opinion is below neutral gain increased likelihood of penalties/injury.

No need to outlaw the behaviour, just penalise it. That way, desperate times can use desperate measures, but it's really not something you want to do.

Not a bad idea, but my point still stands. By radically editing my player weights, it would be simple to make positions other than WR or CB low-rated - and those two positions very attractive. Half my players would believe they should be WRs, the other half would believe they should be CBs. Then how would the penalty be applied? Those players would be happy believing that they are playing in the correct position.

This is already happening with tweaker teams. If it can be done, somebody will do it.