Aimee Ponte crushed Di Aitchison in the final of the Intertrust matchplay tournament to become the Guernsey ladies' golf champion.
The 15-year-old won 11&10 at the 26th hole of a scheduled 36.
Ponte, who first became the island champion at the age of 13, will now meet Jersey's Jenny Deeley in the inter-insular on Sunday, 19 June.
Both Ponte and Aitchison are in the Guernsey team for the Island Games in the Isle of Wight.
They will be joined in the women's side by Jan Chamberlain and Abigail Howard.
The junior title was won by 15-year-old James Hamon, who beat Tom Pattimore 4&3 at the 33rd hole.