Remove their phone number from your phone book & recent call list.
Remove them from your instant messenger & email lists.
A good tip is wrapping a piece of elastic cord or rubber band on your palm(or wrist, if you prefer). When you feel like thinking about that person, just pull the cord up and then release it. It will possibly hurt. If you do this thing few times in a week, in a period, than you will probably develop a conditional instinct. Every time you start thinking about the past friends you want to forget you will feel pain and will intentionally stop thinking about them. Use the cord only when you really think that you need help. After a preferred period of time stop doing the procedure. :lol:
Warning!
Don’t do this to people that understand what real “friendship” is. You don’t want to “slam down” innocent people when they actually show they care about you.
Don’t get too addicted to having the rubber band on your palm or wrist it may become a habit, so try not to only rely on that.