Create an Event Handler Feature

1. Start Microsoft Visual Studio 2010.
2. On the File menu, point to New, and then click Project.
3. In Project Types, under Visual Basic or C#, select Event Receiver.
4. Type DeletingEventReceiver as the project name. Click OK.
5. In the SharePoint Customization Wizard, choose Deploy as a sandboxed solution. Click Next.
6. In the Choose Event Receiver Settings dialog, select List Item Events in the What type of event receiver do you want? dropdown.
7. In the What item should be the event source? dropdown, choose Tasks.
8. Choose the An item is being deleted option in the Handle the following events list. Click Finish.
9. In the EventReceiver1 file that is created, insert the following code in the ItemDeleting method.

properties.Status = SPEventReceiverStatus.CancelWithError;
properties.ErrorMessage = "Deleting items from " + 

properties.RelativeWebUrl + " is not supported.";