Love this topic when it comes up!!!!
I only play games on location, and deal with this often. just kindly let them know that you're on track for the grand champion score, and simply don't have time to watch some coil burnin double flipper struggle to get a mode goin. Then they put tokens on the glass, to this I reply: I appreciate the encouragement, but I'll be able to achieve my goals without your help. Then I throw their tokens on the floor and cradle the ball to watch them shamefully pick them up off the dirty arcade floor.
I cannot believe the audacity it takes to ask another human to relinquish control of a pinball table.