The SIM card in your mobile phone stores information and identifies you to your network. If someone gets a hold of your SIM card, they can extract information from it and possibly use your mobile phone account. You may deactivate your SIM card because you are switching providers. In some cases, your SIM card can become worn over time, causing you to replace it. In either case, you must contact your mobile service company to deactivate your SIM card and then dispose of it in a way that prevents your information from being extracted from it.
Call your mobile service provider from a phone other than the one using the SIM card in question. Explain your situation. Most service providers will be able to deactivate your SIM card within minutes over the phone.
Remove any information from your SIM card, including pictures or your address book. Navigate to the SIM card menu of your mobile phone and delete all of the information present.
Cut your SIM card in half with a pair of sharp scissors. Dispose of the SIM card by throwing the pieces away.