Is it legal for a health insurance company not to let me cancel the expensive medical plan during open enrollment?

I have not heard of plans that cannot be cancelled but health insurance is a specialized area of law and perhaps there is something in the Affordable Care Act which governs this or if the plan is through your employer it may be part of the contract. Start off by reading the contract. What does it say about cancellation? What if you simply do not pay the premiums? Why do the premiums have to be deducted from your pay pre-tax? Won't you be cancelled if you do not pay? Do you have other insurance? I would talk to the Human Resources/Employee Benefits person at your employer and get the answers to these questions. I would also seek to rescind the pre-tax withholding.