I’d put it down to selfishness. People are more concerned with solving their own problems than with how much they are inconveniencing other people. The angry mail would be people who people with an inflated sense of entitlement, the kind of people who will talk at length about what their “rights” are but don’t consider their responsibilities.

I wonder too if you are coming across cultural differences. Of course it may also be that some of them are relatively young and their social skills are not yet developed. It’s hard to tell how old someone is from an email, and there’s a tendency (at least I’ve noticed this in myself) to assume that people are exactly like you until you are proved otherwise. (I am always surprised to find out that people on a forum are significantly younger or older than me, or not male.) So maybe it could also be that people assume that your interests align with their own, or they are surprised to find out that they don’t.

Rules will always fail

I’m not convinced that’s true.

I’ve tried with the “emails about _____ will be ignored� warnings on the contact page, but they were ignored, so to make the page cleaner I just removed it.

Do you mean that they were read and ignored or simply not read? I’d say it was the latter because:

users don’t read anything

So it’s not so much a case of “Rules will always fail” as “people will ignore rules until they are told they have broken them” and your system has to take that into account, which is why I suggested warning them only when they have tried to message you about something off topic, rather than before. By presenting the warning to them when it is relevant they are much more likely to read it and act appropriately.

You say that Account/Invite Requests are the most popular. It shouldn’t be too hard to pick out some words/phrases that would reduce the spam from the contact page somewhat. It’s a pity you can’t hook up to some kind of bayesian filter. Although none of that will do anything about direct mail.

Ultimately what I’m proposing is that anyone who wants to mail you having to deal with your spam filter rather than you.

Ultimately the question is: Why do people feel compelled to do this?

You’d need to be careful with how strict it is in terms of what will trigger it and the language you use for the warnings because you don’t want to put people off sending on topic mails.