What you describe would be off topic. You should report those posts and they should be deleted.
Yes, but that only applies for that specific scenario. A small change could avoid that:
The scammer could ask who can sell gift cards, and then his alt could offer their services, posting an auto-buy link.
Other alts could accuse the OP of trying to scam (to justify the warning at the top of the thread) and vouch for the first alt.
All of this while the thread is locked and nobody else can intervene.