Gmail has a great spam filter, but the spam is hard to delete. And that's a problem if you get hundreds of spam messages every day. Gmail is rolling out a new link called "Delete all spam messages now" so you don't have to repeatedly select all messages and delete them. If you don't see the link yet, you'll find there in the next days. You can see it in the screenshot above, that also shows the previous design.
Clicking on the link gives this message:
Don't forget that Gmail allows you to create an infinite number of bulk addresses like email@example.com. You should use them when creating accounts for different services or forums. If you see spam coming to that address, set a filter that moves all that messages to Spam.
Update: there's also a link that allows you to empty the trash.