[also, preventing your friends from seeing that you've played Olivia Newton John's Xanadu 57 times ... Um. Hypothetically, obviously.]
1) Click on the song you wish to banish to the outer reaches of itunes
2) Right-click
3) Fourth option down is "reset play count"
4) Play song a couple of times for that "oh yeah, I just downloaded it for a laugh" excuse to ring true.