While that's a valid question, even if OP successfully defends himself, there's still the issue of marlboroza not being active. I guess someone on DT could leave a counter-feedback, but OP really ought to have spoken up when the accusation was made.
I don't think counter-feedback is still valid with the current trust system, other user can leave a neutral feedback, but still it doesn't clean off the negative feedback.
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Yes, it doesn't. IIRC the old trust system works like the sum of trusts given to an account, e.g.: someone with three positive and two negative "trust" will shows +1/0/-0, while someone with three positive and three negative will show +0/0/-0. The system today works by showing the sum of each feedback, e.g.: someone with five positive and two negative will shows +5/0/-2 instead of +3/0/-0. So the counter-feedback which on the "old world" can be and were used by DT members to negate an arguably invalid negative trust, it doesn't work on this new system.
The best OP can hope is for the DT members to distrust Malboroza. Enough distrust will kick him from DT and thus will make the -1 score "invisible" from OP's default profile. But given the multiple posts made by several users here on his findings and the reason behind the feedback, I can't see why we should start ~malboroza.
OP, if what you said is true that you're only looking to have a deal with someone and you think that malboroza's tag will hinder you from a successful deal, use
hugeblack's and
dimonstration's advise to use an escrow. It'll "bypass" the cautiousness raised by your counterparty due to the negative tag you have.