I don't think this requires a rule all it's own.
"No going on about annoying things no one cares about". Yeeeaaahhh...no.
Instead when someone comes into the shoutbox and says "Does anyone here like the Jonas Brothers?

" then just simply say "No, sorry, I don't." and that's that. Question answered.
If the person asks why then tell them.
Example:
"Aww, I love them!! Why don't you like them? ):"
"I don't like that sort of music"
Sorted.
IF they instead are going "Any1 here lyk the JOnas Bros!?

" then report them for spam, particularly if it's one of those people that keeps going "ANY1???

" because that's just plain spamming and is against the rules anyway.
If instead they keep on asking everyone repeatedly if they like them or keep going on about them,
Example:
Person1: "Anyone like the Jonas Brothers?"
Person2: "No sorry"
Person3: "No, I don't either"
Person1: "Awww, why not?!"
Person2: "I just don't like their type of music"
Person3: "I just don't .__."
Person1: "But they're so awesome!! You should listen to them more! You'd soon like them!"
Person2: "I don't want to thanks"
Person3: "No thanks"
Person1: "Jo Bros Rock!! 8D Go listen to this song!!! *links one*"
Again - more spamming. If no one's interested it needs to go no further. Beyond that just
don't reply to them, get on with your conversation and don't flame them!.
A mod will get there soon enough and be able to deal with it.