To disable an alert on a dashboard, go to the particular dashboard from the My Dashboard page and click on the Configure Board option from the left pane.

Select the "I do not want to receive this alert" under the ‘When Do You Want To Receive This Alert?’ sectionand click on "Save and take me to my board".

Alternatively, you can go to
​User Menu (on top-right corner of the Dashboard) > My Alerts > Enabled Alerts

Select the Action Icon next to the alert you want to disable.

This will open the ‘Edit Alert’ page. Select "I do not want to receive this alert" option and click on "Save"