there is an unpleasant spice in my purchased frozen italian meatballs from Costco. What can I do to lesson it.

Make your own and freeze them. It is SO easy.

To rectify what your have currently, you could thaw the uncooked meatballs and add it to more ground meat to dilute the spice and possibly add more ingredients to mask it. At that point...you might as well just make your own.

Kept in an airtight freezer bag, I can imagine one could keep them for at least a month.

You can always take them back to Costco with the receipt.