Do you know what is considered reasonable in attempting to stop such a sitution?
I guess that's where at least part of my concern lies.
I.E. will you be facing a false imprisonment or battery claim if you have to physically restrain them? Or is it enough to call the cops after they leave.
This is where it gets complicated. It's difficult - no wait, it's foolhardly - for me to try to give information which crosses jurisdictions and which attempts to cover all situations. I'm not a lawyer but any lawyer will tell you that there are no such things as "cut and dried" or "open and shut" cases. As they say, each case must be decided on its merits. And importantly different jurisdictions take different views and stances on nearly everything.
But there are some common threads.
Speaking from my jurisdiction - and only from my jurisdiction - if you have a duty of care and you discharge that duty of care in good faith and in a reasonable manner you won't be criticised by the courts. But there's only so much you can do. If I have a party at my place and one of my guests gets off his/her face and I know or suspect they are going to try and drive I will do my best to avoid that situation. What's open to me?
I can try to persuade them to stop drinking.
I can take the liquor away from them but no way am I going to get into a brawl with them over it (never hit a drunk, they can fall over and suffer serious or even fatal injuries).
I can tell them they can get off their face but they have to crash at my place.
I can try to persuade them to get a cab home.
I will definitely grab their car keys and hide them from them (surreptitiously of course).
I will try and get through to them that they are being idiotic and that they will get arrested if they try and drive in that state.
In short I will do my best to prevent them from driving under the influence but if in spite of all this they continue to do so then I've done what I can do.