In the Zmodo app, you can delete a device from your account by pressing and holding on its thumbnail on the home page. A pop up will appear asking if you are sure you wish to delete the device. Press "Ok" to go through with the deletion.
You can also delete a device via the Zmodo web app. Log into user.zmodo.com and go to "My Account". Mouse over the device and click on the trash icon that appears to delete that device from your account.