If it is your cellphone, save them as a contact on your cell phone, then set their default ringtone to "No Ring" When they call it will not bother you.
If it is your home phone, call your home phone provider and tell them you want this number blocked from harassing you, the caller threaten your child. If they say they can not, threaten to cancel your service and speak to a supervisor.
If that fails there is free software called phonetray . com that will leave a service disconnect message for any caller you want. Just put in their number and when they call the phone will not ring and they will hear the message. All you need is a duplex 56K modem. Check out their site for details and help setting it up