How to invite users to answer questions without encouraging non-answers?

In various contexts, websites try to boost activity in user-driven question-and-answer sections by actively inviting users to participate. Stack Exchange does something like that with the statement

Know someone who can answer? Share a link to this question via email, Google+, Twitter, or Facebook.

Amazon, on its Q&A section, sends out mails to at least some previous buyers of the same product, asking for whether they have an answer, with a mail that reads something like this:

As someone who owns , can you help this fellow customer?

Unfortunately, the latter seems to attract answers whose writers know do not answer the question. Typical examples include:

• I don't know.
• I am not sure which product you are referring to.
• I believe it will, but I do not have this product.
• I have a different product, and it works there, so maybe it works for yours, too.
• Sorry, I can't answer this.
• I have researched online because I do not have this product, and it seems it is possible.

Obviously, these answers are not very useful. At best, they are not any better than what a random person could have told you (irrespective of whether or not they own the product the Q&A section belongs to). The primary problem appears to be that users feel compelled to answer even if they do not have an answer.

How can a system reliably encourage users to participate, but only exactly those users who actually have something to contribute to the particular problem at hand?

Answers I could imagine point out specific ways to phrase the invitation mail (that makes clear that if the user doesn't have an answer, they can just ignore the message), or maybe a more promising system (or user pre-selection) altogether.