You can delete your team on the website by going to your “My Team” page in the top navigation.
Click on the 'Edit' tab to bring up the page that allows you to edit your team details. If you don't see the option for 'Edit' this will be because you are not the 'Team Leader'.
At the bottom of the edit page, you will see a link to "Delete Team" that you can click on and follow the instructions on the screen.
To delete the team, you need to the the owner - i.e. the user who created the team - and the challenge must not already have started.