[borski]

A lot of times, you can't. They literally will not discharge you until you can prove you have a viable form of transportation, in the form of a friend or family member who can look after you, or a medically-trained taxi. Until you've set one up and they have released you into their care, they will not let you leave.

[AnimalMuppet]

I don't think they'll physically stop you from walking out the door, though (that's kidnapping, in fact). But as rconti said, they'll make it very clear to you that you're leaving "against medical advice".

(I suppose they might physically restrain you if they have reason to think that you aren't mentally competent at the moment...)

[robryk]

> that's kidnapping, in fact

Nitpick: false imprisonment, not kidnapping (kidnapping requires movement).