There's this cause "You were scammed or you strongly believe that this person is a scammer.", so a negative feedback is certainly sanctioned. 
OK, so it changed again. Yesterday it just said "You were scammed".
Problem solved then I guess.
Yep looks like Theymos read it and fixed it
Neutral will just exist for non-impact ratings I guess where one won't receive positive trust or negative trust.
But agree that a suspect categorization would work better