The title says it all. Say one of the forum user seems fishy. Assume, he has started a probable scam ICO. Can I create a flag for him? Sending negative tag other than DT members doesn't work.
You can create a type 1 flag if you believe it's appropriate (according to its description):
"Due to various concrete red flags, I believe that anyone dealing with this user has a high risk of losing money. (This flag will only be shown to guests/newbies.)"
You can also just create a thread or ask a DT directly to tag him.