"But I've always been able to deal with this by pointedly reminding the most demanding people that I'm not at their beck and call. Most of those companies who mistake me for a contracted vendor are prepared to recognise their mistake once I point it out, and the more self-aware ones even apologise. I've not even found it necessary to be especially rude: a plain statement of the facts of life normally does the job. If one of them is rude to me, then the quintessentially British approach of a faint frown and a tone of mild reproof (or its email analogue) generally gets good results – probably a lot better than mouthing off like a sweary 13-year-old in return."

It is easier if you are a native English speaker, because you have a wide range of expressions at your disposal to express disdain or irritation without going nuclear.

It is also easier if you are the single author and not fighting with others in the same code base.

So I don't think easy in general to react maturely, especially if you have made large contributions to a shared code base and some pushy person comes along and makes demands (sometimes the impolite person is another contributor, which makes things worse).

EDIT: Perhaps you meant that the person making requests should not be a jerk, in which case of course I fully agree.