I don't think you can terminate service on a line via MyVerizon, so you'll probably have to call *611 and ask that the service on that line be ended. If that line is the primary line on the plan, you'll want to switch it to being a secondary line first because cancelling the primary line can cancel the entire Family plan.
The only way to cancel a line of service is to call in to customer service and request it.
Good advice from catfish about the primary/secondary line consideration....
When you remove/cancel a phone from your family plan - check to ensure they cancelled it and did not suspend the service. Unfortunately, I cancelled a phone via customer service, was willing to pay the early termination fee an instead they suspended the phone. The phone is no longer under contract and I am still being billed and I just noticed it is listed as suspended again. You really have to stay on top of them to ensure you are getting what you requested.
The ETF for a smartphone starts at $350 and drops $10 for each month of you contract you complete. Basic phones would be half that($175 start, $5/month decrease).
Additionally, whether or not you have a smartphone or basic phone ETF depends on which type of phone you purchased at your last contract signing, NOT necessarily on which type of phone you currently have active on your line. For example, if you last purchased a smartphone when signing a contract but now have a basic phone active on your line, your ETF would still be that of a smartphone.