Sometimes you are able to hijack the car but often times not and blocking the car if they don't stop for reds is only going to make them run....
What my friend and I do is we trail him until we get on a open road with two lanes and then pace him and I then aim my gun using the Aim button before I fire so one shot puts him out the best gun to use when doing this believe it or not is the SNS Pistol it doesn't damage the car but will pop him dead.
I like to do the Simone missions by myself when there's nobody else to do missions with and 90% of his missions are able to be done with one person but some missions like a double repo or chasing the tornado repo you need two people you can do the running repo by yourself I've done it then you can do the single car repos and destroying rival dealerships easyily done by yourself and only takes 5 minutes when the Police are called on you just call Uncle Lessie and get the wanted stars removed and you're home free you can do this with every simone missions but it will only work if you are by yourself in the car if you have a rider or you are riding it won't work because he'll have the stars and if another person has them then you'll get them.
If you do Simone missions on 1.5xp ypu can get between 1,500 & 3,500 xp per mission on hard.
I personally like his missions just wish they would add more missions though I've played all of them at least twice some more then that.
Another tip when you are doing the mission where you have to destoy other dealership cars go and get a Buzzard if you can you'll be done with the mission in 3 minutes and get 2,000-2,500 XP or get a Heli to make a fast getaway a heli is a good get away car in every mission.
If you are level 30 & higher you can call Merryweather and ask for a Helicopter pick up and they'll send a heli right to your feet and you can either shoot the pilot and take the plane sometimes getting one star or get in as apassenger and set a waypoint for him to go to but he won't land after he takes off so it's better to shoot him and take the heli yourself.
Merryweather will be mad at you but after 10 minutes all is forgiven and you can call again.
I don't understand why Merryweather will fly to you for $1,000 but Pegus won't bring you $750,000-$1,750,000 heli to you.