I actually had to make this decision last week! I had an interview scheduled for 2/25-2/26, but I knew that I was already going to commit to my first choice. It may seem like it's a bit rude to decline the offer, but it is probably best not to waste their time (or yours) if you do not plan in choosing that program even if you were to get in. In talking with the faculty member that I work with in a clinical lab as well as some grad students, I felt it best to kindly let the faculty member and grad student host know (on 2/21) that I'd accepted another offer and would no longer be attending. I have not heard back from the grad student (and honestly don't expect to), but the faculty member whom I applied to work with replied with congratulations on accepting a position and a kind note thanking me for letting her know. Hope this helps!