Are you attempting to remove the phone from displaying in Sprint.com, or are you trying to remove it from your account.
If you have set up your account as an account admin (using the account number). It is going to display all devices on the account. You would need to create a new Sprint.com account with the specific phone number you are wanting to manage separately.
If you are trying to remove a device from your online profile, there are a couple of steps that you need to do. This is, assuming you you a device attached to your profile. Note that you can only associate your online profile at a time:
Go to My servcies and device tab
In the upper left hand corner, click the link under your device "Change device on profile"
Select your preferred option. See attached below, but you can choose to remove it complete here.