If you right click the select mode arrow you get a context box where you can indicate what kind of objects you want to be able to select. It looks like this:
![[Linked Image]](https://i.imgur.com/xadHLb6.png)
Disable the terrain selection and you will no longer be able to select it by clicking but you can always select it through the terrain interface. Unfortunately this does not persist through restarting of the editor so you will need to reapply it next time you open the editor if you want it to behave this way again.