I think there currently is no way to ban certain songs from the songrequestlist or certain users from not being able to request other songs.
I usually moderate the songlists of ONSCREENlol's stream, and there is no way for me to add certain songs to a blacklist (ear rape, sanic etc) or even certain users who keep requesting those songs.
I'd like to see a command like !blocksongrequest [url] and !blockusersongrequest [user].