Although a complaint could likely be filed about gold purchased after an shadow-ban, most likely a refund wouldn't be rewarded. Often sites will claim that the premium features were purchased for an account and it is up to the account holder to not behave in a way that would result in a banning.

In short, it may be worth issuing a complaint, but it likely wouldn't result in much, unless the user could claim that gold was purchased without knowledge of a ban already in effect and a refund was refused after the transaction took place.