Hi,

playing Conflict at the Carrock on nightmare mode.
Is it legal to use Thicket of Spears so that Morris cannot attack, if he is engaged with me?

You can use thicket of spears but, even though the rest of your non-immune engaged enemies will be affected by it, Morris will not.

I didn't find anything on thicket of spear not affecting immuned enemies. Did I miss it in the faq or is it in an official response ?

FAQ 1.47

Cards with the text "Immune to player card effects" ignore the effects of all player cards.

So Morris ignores the effect of thicket of spears.