First, a negative feedback from you doesn't have much impact (if any), as you have negative trust score.
Also, if someone want to annoy another person with negative feedback, he could register new accounts for the purpose instead of paying you.
I imagine OP isn't so stupid, he's planning on using that account. Anyway, trust abuse is pissant work, and every dumbass, scammy piece of shit thinks it's above him. No offense meant toward OP & clients.