Blackball.

About Blackball

A blackball is a flag on a lifeguard tower or other highly visible area to show surfers that they must clear the water. A blackball flag is represented with a black circle in the center of a yellow or red flag; a yellow background being the most common. The flag is designed to protect swimmers from potentially harmful surfboards that may be moving at velocities after a "wipe out" from a wave. Swimmers maintain that the policy is a safety measure, while many surfers contend that blackball is an attempt to cull the in-water population in certain zones, providing preferential treatment to swimmers over surfers.