See thread title.
Preventing your users from transferring their domain names to another registrar, due to billing issues with services separate from said domain names (such as use of your web servers), is unethical behavior on your part and you need to correct it.
And if I am not mistaken in my interpretation of this document:
… you are also violating ICANN’s domain transfer policy by doing so.
Note the lack of the following reason that a registrar may obstruct a transfer: “Registered Name Holder did not pay Registrar for hosting domain’s content on a web server(s)”. Everything is about the domain name and obstructing a transfer for reasons unrelated to the domain name itself violates the policy.