Rather than using javascript, wouldn't it be easier to use a user CSS rule in your Display Settings for this, such as .chatfrom_52855 { display: none; }? That would hide the chatter everywhere, such as in the chatterbox nodelet, fullpage chat, and last hour of cb. If you want to be more specific, you can make it #id-596792 .chatfrom_52855 { display: none; } to hide it only in last hour of cb.