I was reading recently that some Natural Doctors believe that it takes about 6 months of off and on treatment to get rid of worms.
A regular MD can prescribe medication that will kill pin worms.
Re-infection is a problem if EVERYTHING (bedding, etc.) isn't washed in really hot water every day. And if everyone in a household doen't take the treatment at the same time.
Not eating sugar and sugar products until the treatment is completed is reccommended for getting rid of pin worms.