I am getting repeated e-mails for a position as a secret shopper at Walmart, it sounds like a scam to me, is it legitimate?

I agree, its a scam. Don't open any links in the email, block them.

Unsubscribe from the email list sending them

No

No such thing if there is a job like that will only go internally

I wouldn't trust those emails. Block the sender and delete the emails. If Wal-Mart wanted you, they'd mail you a letter, or call you on the phone.