Probably.

Even though everyone who participated in the contest is basically asking to be "scammed" (for their research), I'd say that leaving a negative trust rating would be overkill in this situation. There's a decent chance that the OP is going to actually pay.

But here's what you should do, instead of just negative repping him.

1) Someone (preferably on DT) PMs him to accept escrow.
2) If they refuse without a valid reason, neg rep them.

Done.