To disable this service, you can contact the customer service center of your mobile operator. Do not forget to bring your passport, your identity.
You can also call the company supports you cellular. Operators will ask about the owner of the SIM card and then turn off roaming.
If you have the opportunity to access the Internet, you can disable this service on the official website of your mobile operator. For this you will need to enter the password and phone number. And in the opened window find the tab "Services" and opposite values "roaming" click disable.
If you don't have the password to use "Service guide", you can get it. About obtaining you can find also on the official website.
You can also disable roaming via USSD-commands. The combination of the values you can see on the official website and in the booklet that you have provided when connecting to a given operator.