While you can mute and dismiss Web Alarms from the Web Alarms popup window, you cannot disable, or turn them off, from here. Instead, you enable and disable Web Alarms on the web interface on the User Preferences dialog (GO > Configure > Preferences). Also from the User Preferences dialog, you can adjust the Web Alarms refresh interval. The refresh interval indicates the number of seconds WhatsUp Gold waits until checking for new Web Alarms.
By default, Web Alarms are enabled on the web interface with a refresh interval of 120 seconds.