This's why we have the trust system.  If you have a valid evidence (proof of transactions) then leave the user a negative feedback with reference. You don't need any bodys approval or seek any permission to do that..

To make things more clear, do as @chenile have suggested with addition to creating a thread, add some valid evidence (proof of transaction) then other forum user plus DT members will join you in red tagging the account.

Note: If you don't have any vaild evidence of scam, it'll be considered as just an accusation so it'll be best you start your topic in Scam Accusations then other experience users might interval in proofing he's a scammer.