I think there were like two successful attempts for unions to get at Walmart.

http://www.bloomberg.com/bw/articles/2012-12-13/labor-disputes-the-walmart-way

1) Butchers at a Walmart in a small Texas town successfully joined a union. This inspired butchers in other stores to try to do the same thing.
What did Walmart do? Two weeks after the Texas butchers unionized, Walmart permanently closed down the meat departments nationwide.
Shut the whole shit down lol.

2). There was a Canadian Walmart that successfully unionized. Walmart just shut the store down.