There may be no official way, but i could create a barrier (At every bridge) on every mission other than the ones that are past that stage, or the mission where you travel across, and maybe there is a way to put a police barrier (If you travel across, You get four stars?) On every mission leading up to that?
Well, you can put policemen at these barriers equipped with Miniguns. It would discourage them to try to pass, although I don't see a reason why they would try to leave that far from the mission's location.
Load progress of your original game before playing an mission,then go to 'New Game' 'Design Your Own Mission' press 'Y' and load an mission. There'll spawn barriers at bridges,gangs on the streets but there won't be that if you go to second city,police will try to kill you with four stars. I tried it.