I want really to be able to lock the terrain. It never ever should be selectable nor deletable accidentally.
Hey morez, sorry to hear about this, I know how frustrating it can be. You can prevent this in the future by modifying the kind of objects that your "Select Objects" mode can grab. Right mouse click the select objects icon to get a context box and you will see a list of objects that are selectable. Simply set "Select Terrain" to false. I haven't seen a way to save this as the default behavior, so as far as I know you'll need to do this each time you startup.