While I wish I had a way to quickly mark an email ‘Spam’ on my iPhone’s Mail client, even more useful to me would be a button that would hang up on a telephone call AND mark the number as ‘Spam’. That would help everyone by minimizing the amount of solicitation they got.
My guess is that the Apple SDK does not provide any sort of API for plug-ins into the Phone application, for obvious reasons. Something like this would have to come from on high and be coordinated with the phone company.
In the meantime, I’m going to take some advice I saw online and create a ‘spam caller’ with a custom ringtone, and every time I get a spam call I’ll add that number to the same contact on the phone. But it’s more work for me.