how do i block alerts and banners?

Scroll up on the notifications area to reveal the option.

For other settings go to the Notifications preference pane.

You govern the types of contents your Notification Center displays by going to System Preferences --> Notification.

Put your mouse pointer on the black area and scroll up...

I think someone already mentioned this, but you two-finger slide down in the notification sidebar to show that slider. You can also Arrow-Up.