I've been thinking for this kind of thing for quite some time.

Maybe the best is to 'limit' the request both in general and by account.
What I mean is usually the limit of same song is once every 200 song, right?
Make this long limit as 'Account Limit' mean same song can only be requested by, for example, me once every 200 song.
But, make a general limit of, let's say, once every 50 songs.
This mean than I couldn't request, for example, "Sousei no Aquarion" after I request it until 200 songs are passed, but someone else who want to hear that song only need to wait 50 songs before they can request "Sousei no Aquarion."
Then, put all guests in a single account. This way it won't limit guests' requests too much.

Why I think it's the best?
First, a user still can't abuse and spam songs, which become irritating sometimes.
Second, when a user want to hear song 'A' but accidentally someone else was requesting that couple hours ago, this user doesn't need to wait for amazingly long time period (200 songs is indeed a long time period. Period.)
Third, from guests' point of view, nothing really changed, since they are still limited by 200 songs limit after all.

What do you think?