The simplest way is to use the following settings on the app:
Start type = Mass
Record lap times = On
Number of laps = Time-limited
As racers complete lap 1, you’d physically pull the slowest runner from the race - i.e. disallow that racer from starting another lap.
To mark the racer as “Finished” on the app:
Tap “Lap control” toolbar button
Tap “Finish racer” button
Tap the racer on the screen
This will clear the racer from the timing screen. You’d repeat these steps after each lap, until there’s just one racer left.