> What is the best way to warn others about this situation?

You should contribution for moderator attention by using a custom flag.  If you believe the contribution to be spam, you should flag it as spam, if a moderator does not agree they will simply decline the flag.

> The typical user coming to help from this site could easily fall
> victim to the scheme, and my question, therefore, provides an effective
> advertising platform for their product.

Leaving a single comment saying the answer was unhelpful.  You should avoid having a discussion if the other person believes your comment isn't helpful.  If they leave a comment you believe to be rude, you should flag the comment, so a moderator can deal with that behavior.

> Because I could not find how to complain or warn others, I responded
> as you can see here. But I would appreciate someone in authority at
> the site to provide me with a proper way to complain about this
> situation and warn others.

Creating a meta question will also bring the unhelpful contribution to the attention of others.  There is nothing else that needs to be done.