When an answer recommends a product, only recommends a product, but is a relevant and seemingly genuine recommendation rather than spam, is that an acceptable answer? And if not, which of downvoting, flagging, or both is the way to go? Also if not, (how) could such an answer be made acceptable?
Case in point: How can one learn to identify intervals by ear without singing?.
A related topic: What should be done with answers that consist almost entirely of material copied from external sites?