This is perhaps implicit in your comment, but "no" is also an acceptable answer if you explain why. We have a product that we have a relatively clear idea about, where we spend a lot of time making sure we're keeping things simple and focused. I reckon we say "no" to at least one third of ideas and "not for quite a while" for at least another third, explaining why each time in terms of our roadmap for the product. We'd found that customers tend to respond quite well, and for those where it's a deal breaker, they're probably better served by a different product anyway.