Dead rising zombie genocider achievment help xbox 360?

All i need is zombie genocider on dead rising. Well i seen on youtube someone got it by just driving in the park over and over which took them 39 game hours. I went to do it but for some reason i keep running into otis and getting the phone then running into the girl jessie and having the case mission pop up...And when i watch on youtube they have no case missions or phone calls...just the time to get to the helicopter in 72 hour mode. How do i not start any cases and make it outside to drive?