Is there a way I can edit/delete alerts posted by coaches?

No. The athletic director is only able to edit/delete alerts that you have posted.  

To delete an alert, please follow these steps:  

1: Log into getvnn.com and click "GO TO ALERTS"

2: Click on "alerts sent"

3:  Find the Alert you want to delete and click “Edit”

4:  In the “Edit Expiration” area click on the dropdown and select “Expire Immediately”