You can delete alert categories as required. When you delete an alert category, any existing alert rules that are associated with the alert category will have the category removed. However, monitoring and alerting will still take place, but the alerts will appear in the Uncategorized column on the Central Dashboard.
In Service Center, click Configuration > Alerting > Alert Categories.
Open the folders by clicking the chevron (>) and locate the name of the category you want to delete.
Click the Delete icon
for the category you want to delete.Click OK.