You can put a ~ beside their name if you choose not to trust them, for example ~ranochigo.
And just to add to the complexity and controversy, if you ~ someone, it's generally agreed that you don't trust that member's feedback on other members--not necessarily that you wouldn't trust that member in a trade. If the latter is the case, giving that member a negative would be more appropriate (especially if you're doing it to keep track of personally who to trust and who not to).
Really? I've always been under the impression that DT2 was also included there; as adding the ;dt to whatever link you were in did also include feedback left by DT2 members
And here's another example of the massive confusion around the DT system. I've been around the forum for about six years and I wasn't aware of what ranochigo wrote. DT2 members still have a lot of pull on the forum, since getting a neg from one of them will give you some red on your trust score (depending on how you've customized your trust list).
OP, I've given up caring about the DT system since the rotation system started. It used to mean something huge if you were even a DT2 member. Now it's nothing special, really.