Is it possible to lock the node type and if so how? For instance if I want a node to stay smooth it will always stay smooth unless I change it.

In what situation are you finding they are changing without you wanting them to?

If it is when you delete a node, press Ctrl + Del to delete the node without effecting the adjacent nodes.

I see you cannot delete a node and preserve the shape of the curve without the node changing. Because if you lock the node so to speak your curve will change more than you wanted it to.