Wow this was literally me a few years ago with the Walmart card haha. I looked up the credit card phone number on their website, gave them some info such as my name, address etc. and closed the account down.

Not sure which company you track your credit score with but I can also see the first and last 4 digits of each account in my TransUnion report so you might want to check there too for information you can give the rep on the phone.

It did hit my credit score but nothing significant and it bounced back quickly.

I've heard tho, that Walmart credit cards are nearly impossible to remove from your credit report... Closed or not. But have not personally confirmed this.