This is why we have the Add-on Download IDs feature on our site. Just go to My, Add-on Download IDs from the top menu of our site and create one additional Download ID for each site you manage (or, at least, each client). Use that instead of your regular (master) Download ID. If your client misbehaves or ditches you you can disable the add-on Download ID and they are immediately locked out from our downloads, without affecting your other clients.
The reason we do not hide the Download ID is part technical (Joomla! doesn't allow us to do that) and part logic (it's shown in the download URL). So instead of wasting time to force Joomla! to do something that it's not designed to do and which would make your life harder without any privacy/security benefit we invested our time into the add-on download ID feature which is far more convenient.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!