Examine the technical capabilities of your mobile phone. Some models have special functions, with which you can block a number for free. Usually they are available in the settings of the address book.
Use a special trick that you can use on virtually all modern devices. First, make sure that your SIM card is disabled, the voice mail service. Then go to contacts menu and select the one you want to lock. In the settings contact check the box next to "Send all calls to Voicemail". A caller can call your number, but given that the service you disabled, it will constantly hear only a busy signal as if the line is busy. You, in turn, will not be subject to disturbance by unwanted companion. This is one of the easiest ways to block a number for free.
Install on your device one of the special apps created specially for the fine tuning of incoming calls. These programs are usually available for installation on smartphones running Google Android, iOS, Windows Mobile and other mobile platforms. The most effective of them to block the number for free are "Blocking calls", "Ultimate Blacklist", "Root Call Blocker", and some others. You can download them on the website with media content for your mobile platform.
Periodically through the website of your mobile operator. From time to time it may appear information about the new tariffs, providing the opportunity to block unwanted numbers or to connect this option for a relatively small amount. Also feedback to the operator will help you to unlock your room, if you accidentally close the subscriber access to calls on your phone.