Centralize the system and let the admin moderate it. It's obvious we're not ready for Decentralization just yet.
The DT system was already centralized a few years ago, theymos acknowledged this in his post here:
For years I've been unhappy with how DefaultTrust ended up as a centralized and largely-untouchable authority...
Theymos can moderate the DT system even now. If you think someone is abusing the DT system, you can PM him. I don't know how he will react to such a message or if he will react at all, but you, as well as any other forum user, have this option.
Anyway, there are many conceivable ways to abuse the system, but if it happens, you can just shoot me a PM and I'll fix it, probably in less than 24 hours.