But as far as I know, only tags from DT will be visible in their trust. Tags from us that are not DT are not visible. It's useless if I tag it but no one can see it
As I mentioned before, even if you're not a DT member, you still have the ability (and should use it) to provide trust feedback, just like any other forum member. For now, your feedback will be displayed as "Untrusted feedback" on the user's profile. However, if you use your feedback the right manner, following norms and best practices, other members will take notice and may consider including you in their trust lists. This is how you can eventually
earn the status of DT. On the other hand, if you choose not to provide any feedback, there's no reason for people to consider promoting you to DT, since no one can predict if you'll use your powers for the betterment of the community or not.
How do I report this case of ban evasion? I don't know the staff or admin here. Can I report it with "report to moderation"?
Yes, you can make use of the "Report to moderator" option. However, when providing an explanation, you should refer to the specific post or thread that contains the evidence supporting your claim. Another option, as mentioned by lovesmayfamilis, is to use a dedicated thread that you can find in the Meta section.
So it's quite complicated to ban someone. There must be very strong evidence.
Absolutely! Mods usually need to have some really solid evidence before taking such action, and rightly so. It's not a decision that should be taken lightly.