How does a cement company do business with a contractor like that.

I would write that contractor. What a scumbag. Then I would write the cement company and ask them how they can sell cement without payment?

The contractor is a roid and I would not put it past the cement company to being in on this scam. I would call them on it.

If I got no satisfaction I would complain to the BBB, the local newspaper, etc. I would also complain to the local cops... this might be an ongoing racket.