Is it legal to threaten another contractor that they can't use a subcontractor?

There may be a potential interference with business advantage claim, and/or if the subcontractor is a minority there could also be a discrimination claim. The answer can only be determined by a careful review of the facts. This advice is based on the limited facts that you have provided, additional facts may change the advice. We are not providing you legal advice, rather we are responding to your set of facts based on general legal principles. You should not rely on this information without consulting an attorney and providing the attorney with a complete set of facts.