For some reason i never thought of this before.
Tie someone up. Stick something in or over their anus. Nose clip on the nose. Then, tape a one way valve over their mouth. They can then breathe in but not out. However, what they can do is swallow their inhaled air, and it could even make that easier than most people seem to find it. So imagine this. They're stuck in this situation and can't breathe out, so they hold their breath for as long as possible until they're forced to gulp down the air before inhaling again. Then they're stuck with another lungful which they eventually have to get rid of, so they have to swallow that too. So they're forced to inflate themselves "voluntarily", and they carry on until they pass out or you release them. They can't even fart.
Nice idea?
Obviously if you tried it for real you'd have to think about the danger of inhaling stomach contents.
Erm.... No.