Even if you propose that there should also be a way for people to give them positive points to reduce their negative obnoxiousness level, it will still not stop the unfair usage of the system, in my opinion.
What we are doing is trying to introduce a third rank in the forum:
Please think about those who could be hurt more by this system if implemented. The system will favor those who are DT 1 or DT2 members. Imagine an argument between DT members if member A gives member B a negative obnoxiousness user B could give back and if inturn it exceeds that the trust system gets into use. They can both tag each other with their trust meter. But in a case where a member does not have any DT power he might be the one who loses at the end.
The trust system, merit system, and suggested obnoxiousness shouldn't be for fights in the forum. Its expected to be served with good reasons and not a personal conflict between two members.