While you can mute and dismiss Web Alarms from the Web Alarms popup window, you cannot disable, or turn them off, from there. Instead, you enable and disable Web Alarms on the web interface on the Preferences dialog (click your username in the upper right of the application and clear Enable web alarms). You can also adjust the Web Alarms check interval from the User Preferences dialog. The Check every box indicates the number of seconds WhatsUp Gold waits before checking for new Web Alarms.
By default, Web Alarms are enabled on the web interface and are checked every 120 seconds.