Unless there is a penalty, some people will always try it - even if you get stopped, the worst case is your default boarding position. The only way to stop it i

Unless there is a penalty, some people will always try it - even if you get stopped, the worst case is your default boarding position. The only way to stop it is have a separate line ("sin bin"? :)) that the gate agent puts people in when they try to board early, so that they end up boarding last. Or force them check their bags. Humans respond to incentives...