Call the customer services of your mobile network provider. They will probably need to know the unique IMEI number of the handset (to permanently disable it) and your moby number (to deactivate the SIM).
If you don't have the IMEI number they will only be able to deactivate the SIM, not disable the handset. That means anyone can use the handset simply by putting a different SIM in it, not an ideal situation for something you've paid for.