Looks an interesting read. I've become pretty good at dealing with disgruntled customers and just conflict in general. It helps that with things like ID for movies and alcohol, I'm 100% in the right. Have also heard it all before, so know how to deal with different situations.
I think the key is to realise that even though what they're saying may be stupid (to you), they've felt the need to complain or get annoyed for a reason. So you have to be able to empathise, see where they're coming from, but set them straight. Of course there are times when no matter what you say, they really just want to vent at you, so you just have to employ a bit of the 'broken record technique' (repeating the same message consistently over and over - therefore not giving them anything to latch on to) and let them keep going until they get bored.
You do tend to get more than the fair share of special people at a cinema though. Keeps it interesting, always end up with good stories to tell.