Get rid of the -100 penalty for a bad report, but instead of the 25 millimerits for a good report being sent automatically, leave it up to the individual moderator's discretion. That way, the mods can spot abuse themselves; if someone spams them with mostly inaccurate reports or if reports seem suspiciously like collusion (e.g. someone keeps reporting new posts from the same user over and over again) then they can decline to issue any millimerits. Maybe the rule could be that when reporting a post, the moderator that handles it has the option of sending you between 0 and 1000 millimerits depending on how useful/valuable your report was. Then (just making up numbers) original reports can get ~60 and redundant (but still good) reports can get ~20, really valuable reports like malware can get ~300, and so on (some moderators would likely be more generous than others).