How can the Pentagon bar a "civilian contractor" from testifying?
By pulling his security clearance, thereby ending his career in any classified environment. Shaffer already had his clearance revoked over this.
We need Bush to find his balls and as Commander in Chief order the Pentagon to allow them to testify.
"How can the Pentagon bar a "civilian contractor" from testifying?"
I believe, the main lawyer for the case addressed this in an interview I read. First of all, if he does testify, the Pentagon can claim he revealed classified information. Then he can be prosecuted. In addition, his (Pentagon- related)career would be ruined if he testified against their wishes.
Contractors are just as responsible for protecting classified information as military and DoD civilian personnel. If an individual or a company loses a security clearance, they are out of work/business as far as any military contracts. A suggestion from your government security monitor might just as well be an order.