So you just got your new Windows Azure subscription and your testing out all the neat functionality that comes along with it and start deploying some Virtual Networks for testing purposes to see how they work and interact with your machines. When you go to delete the VNET now that you know how you want to go about configuring your environment, you get the message similar to the one below.
First things first, ensure you have read http://msdn.microsoft.com/en-us/library/windowsazure/dn195495.aspx to ensure you have deleted all the necessary objects that are utilizing the VNET. So let's say you have done what was instructed in the article and it's still not allowing you to delete the VNET! One thing to check for is to ensure the Point-to-Site VPN connection checkbox is unchecked as this will prevent the VNET from being deleted.
After you uncheck this box and then Save the changes, you should now be able to go and delete the VNET as long as all other resources have been removed from the VNET as described in the MSDN Azure article mentioned above.