Q: How to remove a device off iTunes

You can deauthorize all devices once per year whether they are in your possession or not. Otherwise, they need to be in your possession.

You can de-authorize individual computers, but only by using those computers. The only other option is to "de-authorize all" from your iTunes account.

  1. Open iTunes on a computer

  2. From the Store menu, select "View my Account..."

  3. Sign in with your Apple ID and password.

  4. Under "Computer Authorizations" select "De-authorize All".

  5. Authorize each computer you still have, as you may require.

You may only do this once per year.

After you "de-authorize all" your authorized computers, re-authorize each one as required.

If you have de-authorized all computers and need to do it again, but your year has not elapsed, then contact: Apple - Support - iTunes - Contact Us.

For more information on authorization and de-authorization: iTunes Store- Authorize or deauthorize your Mac or PC.

What do yo remove by remove?

Maybe this:

iTunes Store: Associating a device or computer to your Apple ID - Apple Support

or this:

Find My iPhone Activation Lock: Removing a device from a previous owner’s account