Cutting sugar in savory butternut squash dishes
Ruth Lively at Fine Cooking has a similar problem and suggests:


  
  Orange or lemon juice (or zest) adds brightness and zing. A splash of    vinegar helps, too.
  Tomatoes, which are both sweet and acidic, make a bridge for    full-flavored squash dishes.
  Sharp cheeses like Asiago, Parmesan, feta, and goat cheese lend a    salty note and highlight the nutty side of squash’s flavor.
  Robust herbs, such as sage, rosemary, and thyme, work with the earthy    qualities of the squash, yet still offer an assertive
  contrast    to    its sweet side, too.
  Bold spices like cumin, coriander, nutmeg, mace, cinnamon, ginger,    and curry have a natural affinity to squash, making the perfect
  bridge from earthy to intriguing.
  


You may also consider substituting a differing squash; Royal Oak Farm Orchardhas a nice guide (pdf) to many squashes and includes sweetness among the descriptions.