You can't hide instances from an imported module. See for example: Explicitly import instances
It looks like the "overloading" you're trying to do is to allow (==) for other types, like trees. This is easy! Just simply create a new instance:
data Tree a = Leaf a | Branch [Tree a] instance (Eq a) => Eq (Tree a) where (Leaf a) == (Leaf b) = a == b (Branch a) == (Branch b) = a == b _ == _ = False (You could also just derive the Eq instance)