I understand that they can ban me and it's private property. But how can they impede my future employment for not breaking any laws? This has to be a violation of my rights.
They cannot. Simple as that. At most they can deny you the ability to work at the store, but they cannot enter something against your record since this isn't criminal inn nature, it is solely a private issue.
Also, I'm under the assumption that once I receive my receipt, the transaction if finished. I have a membership to Costco and I sign an agreement that states I will show them my receipt and they have signs posted. I didn't sign anything with Walmart and there are no signs posted.
My apologies but that is incorrect. So long as you are on the premises, the store can exercise their privilege, question you, and detain you for a very short and reasonable period of time. A receipt signifies an end for that particular transaction, but not to the privilege. And the store still has the right to double-check before you leave if the transaction was accurate.
Is there a law you can point me to showing me that I have to show anyone my receipt?
I do apologize but this is 'common law', meaning that it is unwritten. The right to be detained by the store is common law that comes to the US all the way from old English law and has been in existence since then.