Some companies, so-called third-party providers, offer you the convenient option of simply paying in apps or websites with your "cell phone bill". For example, in some parking apps or in local and long-distance public transport. This system is actually regulated. Nevertheless, there are loopholes that allow this system to be exploited.
If you want to set up such a block, you can contact your provider online or by phone. Many offer this option on their websites. Bear in mind that you will then no longer be able to pay for some services using the "Mobile phone bill" function.