As an instructor, I regularly give trial flying lessons to people I don't know.
Most people seem to be OK but I do sometimes find myself couped up in a small place with people who I may not choose to associate with otherwise. I do sometimes wonder what I would do if one of them didn't do what (s)he was told.
This is what I think I would do :-
1. Explain that they are compromising flight safety and they must be quiet. I am the captain and they are legally obliged to comply with my reasonable requests.
Hopefully that would do the trick. If not...
2. Explain to them that if they don't do as they're told then the flight will be terminating at the nearest airfield as soon as possible.
If that doesn't work...
3. Tell them that by not complying with my requests, they are effectively hi-jacking the aeroplane, and that I shall arrange to have the police meet them at the nearest available airport.
and..., if that doesn't work...
4. Hit them over the head with the fire extinguisher.
I have to add that in 3 years of part time instructing, I've only ever had to get half way to stage 1.
I would also apply the same to friends who are travelling with me on a private flight and, since I have a choice who I take with me on private flights, I wouldn't take anyone for whom I thought that there was even a remote chance that they wouldn't do as they're told.
Wether they're excited or not, that's just the way it's got to be - anything else isn't safe.
Hopefully, with your passengers, it's just that they don't understand the seriousness of the situation - not that they actually want to be a pain. If so..., then you need to make sure that they do understand - probably in a pre-flight briefing and then perhaps a reminder or two in the air. If that doesn't work then they shouldn't be there - so treat it accordingly.