Guess I am different from the people who do that to peoples' food. I am a laid back person (was at 17 as well). "Bad customers" like that for the most part don't bother me because I know their anger has nothing to do with me and everything to do with them. Fine, they can be angry in general or at a policy they don't like and take it out on me. If you can't take people like that then you need a job not dealing with people. And in 2 min they will be gone anyway, will still an idiot and I could give 2 sh!ts what they think (especially if I am 17 and working in a pizza place).
And they could try and get me fired all they want, becaue every boss I have ever had has never had to question me or my work ethic so, again, the customer could say whatever they wanted because I (and my bosses) know it has nothing to do with me. I have always been confident in those situations.
And, if for some reason, it came to where I got sick of the job, rather than staying and spitting/pissing/spludging in someone's food, I would just find a new job. It's a pizza job at 17; there are a million of those jobs.
Basically, I am saying spitting/pissing/spludging in someone's food is a stupid, nasty, weak solution to the prob and make you a huge liability to anyone you work for. And karma sucks
Thats all I have to say on the matter. You're free to disagree.