Shoutbox

Why do you want to empty it? They delete themselves (not trash or archive, just delete forever) once they get old anyway. My spam box usually has about 50 or 60 spam emails sitting in it, as the new ones arrive, the old ones disappear.

User JavaScript may be a solution. I think it would be either really easy or really difficult to create a user JavaScript to empty your spam, both because of Gmail's massive usage of JavaScript. I wouldn't know where to start writing a suitable script though