Save the number as "Do not pick up" and whenever they give missed call just ignore it. When you're sleeping put the phone on silent so they can give as many missed calls as possible.

If u want to have some fun note the time they give the missed calls maybe for a couple of days and be ready when they are about to call and pick the calls so that they will get charged for the calls they're making. Just pick the call and leave the phone on a table and do ur work. As soon as they disconnect be ready for the next call n do it again. Once he loses some money he'll get the point.

I dream of a better tomorrow where Chickens can cross the Road without having their motives questioned - Unknown