Generally, it is your responsibility to cancel all recurring charges before closing an account.
The original agreement to charge the account was made between you, as the accountholder, and the merchant. You probably provided the merchant with written authorization to debit your account. Because the credit union was not a party to that agreement, they cannot cancel it for you. You need to instruct the merchant to cease debiting your account.
If the insurance company fails to comply with your instructions to cease debiting your account, contact the credit union regarding its procedures for placing a permanent stop payment order against the insurance company.