I'm a Northern Girl
When you enrolled in the health insurance program, you entered into a contract with your employer to pay a certain portion of the premium. You don't have to have signed a separate document agreeing to pay; you knew going in that there was a cost associated with it. The documentation you've quoted makes it clear that you were expected to continue to make your contributions. You can try to get out of paying them and gamble that they won't take any further action but they are on firm legal ground in insisting and you, frankly, don't have a legal leg to stand on based on what you've posted.