What I would like (and what was discussed in iasenko's thread he linked to) would be an indicator to show if I have already reported a post. Most of my "bad" reports are from unintended duplicate reports for spam. Once you've reported enough barely coherent one or two line nonsense, it all blends in to one, and I find myself forgetting which posts I've read and reported before.

In terms of a warning for the user, I would like that too but only if it was actually backed up with some sort of temporary ban. There are several users chronic spammers I have reported dozens of times, and no further action is ever taken against them.

suchmoon's script ([HACK] One-click mod report, not for the faint of heart) essentially provides this, albeit it entirely customizable so the user can define their own buttons. Perhaps some optional forum generated ones would be a nice way to encourage people to start reporting, who otherwise may feel intimidate or unsure about the whole thing.