There is a TV show on Discovery called "Myth Busters"
They tested the potato trick on TV (as well as bananas, thanks to Eddie Murphy in Beverly Hills Cop)
and neither workd. The potato got shot out of the pipe quickly.
Back about 30 years ago, there was an anti-theft device for cars. It was a tailpipe cap held on by wing nuts. The idea is after you parked your car, you could screw the cap on the end of the pipe in seconds and immobilize the car.