Extensions (free and commercial) are a necessary, valid, and valuable part of the Magento developer community. They often offer a complete or substantial solution to developer and user needs, so it's entirely valuable to ask about them here or (more appropriately) inform question askers about the existence of some extension which solves the problem.
That said, mentions of these extensions, especially when they are commercial, are subject to the same standard as any answer. It's never sufficient to link to an extension and say only that "this will solve your problems". Once a poster here does this, if these posts seem to be the only ones they make, and if the linked extensions belong to one company or use affiliate links, they should always be deleted. I have just done this with user alan's posts.
As for what we can do: I've just suspended the user for thirty days and sent him the following message which is derived solely from a StackExchange moderator private message template:
I'm writing in reference to your Magento Stack Exchange account:
We noticed that a substantial proportion of your posts seem to exist only to promote your product or website. Per the help center:
Be careful, because the community frowns on overt self-promotion and tends to vote it down and flag it as spam. Post good, relevant answers, and if some (but not all) happen to be about your product or website, so be it. However, you must disclose your affiliation in your answers. Also, if a huge percentage of your posts include a mention of your product or website, you're probably here for the wrong reasons. Our advertising rates are quite reasonable; contact our ad sales team for details.
Any type of "astroturfing" promotion is not acceptable. It brings down the overall value of genuine recommendations for everyone on the site.
If you can stay within the above guidelines, and offer questions and answers of genuine benefit to the community which happen to mention your affiliation or product in context -- and with full disclosure -- then your future contributions are welcome.
Magento Stack Exchange moderator
I am very reluctant to stifle open communication, especially of problematic behavior, as open communication can serve as an example of others, and transparency is important, but it's evident from his posts that he is shilling for the extension vendor and not engaging any of the comments he's seen so far.