Ideally, you should use such requests as an opportunity to help you and users better understand the application.
If you think of it, the very reasons why you prefer to ignore these requests are quite important information and you'd rather have them stored and documented than buried and forgotten deep down in your mind.
If a request is ignored because you don't have time to implement it, but generally looks like good idea, you better keep a knowledge of that somewhere. Later on, when you have time, you may get back to it and reconsider.
Or, if a request is ignored because it is really bad idea and you can write down an explanation for why it's so, this would also be useful knowledge to store somewhere. Doing so will make it easier to address similar requests from other users, or even help yourself if you eventually forget why you thought it's a bad idea.
Keep in mind that spelling out and writing down reasons why some features may do more harm than good will help yourself better understand your application, it's intended use, limitations and strong points.
As an example, take a look at the very Stack Exchange network. Feature requests that are decided not to implement here aren't buried down. Quite the opposite, these are made public, thoroughly analysed and kept for further references conveniently tagged status-declined at Stack Exchange meta site.