Hi Folks, I believe there are 3 ways to resolve this issue.
1) You can use product groups and exclude certain variants from being advertised (exclude by item ID).
2) You could delete the unwanted product variants within your Google Merchant Center account (Products > All Products)
3) You can remove the products from publishing to Google within the per-product Shopify channel settings, then use the Google Sheets method to create your own feed for that product and it's variants.
Hope that helps,