Unfortunately, it has been shown that alcohol-based hand sanitizers are NOT effective against noro virus! The one real defense against it is hand washing.
The hand sanitizers are still a good idea and should be used frequently, as they are effective against other types of illnesses. But they are not effective against many viruses, including noro.
See this fact sheet
about noro, published by the Michigan Dept of Community Health. Take note especially of the last paragraph on page 2. Many other documents are available, if you research it on the internet.