This is a really sore subject for me from several aspects that I won't get into, but I've always wondered about this since it happened to me a few times.

There is this mall cart that had those stickers on the register (Visa, MC, Amex, etc...). So I pick something up and hand them my Amex. She said they don't take amex and I asked what that was pointing to the sticker. She said a sticker! Well I got really mad at that point and she said its a mall cart that is leased and it was just on there. I threw my Visa at her and she took that.

Can a store say they take a card, but then not? Shouldn't they be required to remove/cover up the stickers?

if they dont take the card they have to get rid of the stickers and any insignia

Years ago, I used to have a Corporate Diner's Club card (remember those?) and I went to this one restauarant and when I was seated, I asked the hostess if they took DC, and she said, yeah, we take that. When it came time to pay the check, I handed my DC card to the server, and he said they didn't take DC. I told him the hostess told me they did take DC. He walked off and returned a couple of minutes later with the manager. I repeated my story to him. He thought for a second, then said, "As far as I'm concerned, you don't have a check. Sorry for the inconvenience." So, I got my meal for free, even though I was about to dig out my Visa card. I became a repeat customer at that restaurant because they treated me right the first time.

or COVER THE SIGN WITH A SHEET OF PAPER

You're full of good solutions. They could also black it out with a marker.

I assume that the sticker is attached to a counter or post of some sort, so they could also get a new post to replace the one that has the sticker.

Or scrape it off...

Or put a different sticker over it...

Or post a sign over it saying "We do not take AmEx, Sorry."

Great story! That's how customer service is supposed to be. I love it.