This is actually the intended behavior for the plugin, but there's definitely a workaround to get your desired end result. Because the plugin doesn't reflect the "live" settings of the list, there's a way we can "trick" the plugin into hiding that text.
In your MailChimp list, you'll want to temporarily remove the predefined value for that merge field in your list. After you save it with no value, in your WordPress dashboard, go to "Settings" -> "MailChimp Setup" -> "Update List".
This will update the plugin so that it sees no information for that predefined merge field.
Then, in your MailChimp list, you can re-add that value and the plugin won't pull it over until the list is "updated" again.
Let me know if that doesn't make sense, I'll be happy to clarify!