Targeting questions are really easy to resolve actually. You just look at the target bar of the spell being cast. In the case of [mwcard=MW1I27]Steal Enchantment[/mwcard] you can see that it targets the Revealed Enchantment and the New Target that the enchantment is being moved to. That's it. Only those two objects get targeted.
Rarely, some spells will have additional targets listed in their effects, but these are also easy to identify because they always include the term "target". If it doesn't say the actual word "target" then it doesn't count as a target.
So, an Arcane Ward on the Enchantment being stolen would stop it, as would a Nullify on the New Target that the enchantment is being moved to. However, the original object that has the enchantment attached is never targeted and a Nullify on it will neither be triggered nor have any affect.