Can I use the forums to denounce a scam?

Yes, as long as your posts are in line with forum rule #8 . In other words, you can post the emails you received or describe the situation, but no names should be mentioned.

The purpose of avoiding naming names in a case like this is to prevent possible issues with libel, since the names used in these cases are not always made-up and can be the names of actual, innocent, persons. In general the messages themselves are sufficient to warn other translators.