This is usually by design.  Synced banners will display throughout the whole spot break. If there are, for example, 3 spots that play during a break and only the first has a synced banner, the banner will continue to display until it returns to live programming.

Along this same topic, if a synced banner *doesn't* go away even after the spot break is over, this means you do not have a "default image" set for your station. A "default image" can be set by performing contacting your player host - either by accessing their Player Web Admin or by contacting their support department.  Some CDN's, such as Triton Streaming Services, allow clients direct access to manager their own player images while other CDN's require that you contact them to have the default player image set-up.