Yes, you can use category security settings to prevent users from posting to the parent category while allowing them to post to the subcategories. There’s more info on how to do that here: Understanding groups and category permissions (security settings)
No, there’s not a setting for this. When posting to the parent category is disabled, users will see a disabled new topic button:
And when they switch to a subcategory they can post to, it will become clickable:
In the composer they’ll be unable to select the parent category if they don’t have permission to post there, and if they select “New Topic” from the subcategory page — that subcategory will already be populated.
We should consider making some improvement here on the disabled parent category — we don’t make it very clear why the button is disabled, or that a subcategory can be used. I can see why you’d want a default subcategory to be used there.