Put a activation radius sphere (using a collision sphere) aroung your and when the player overlaps with this sphere (he is “close enough”) then you use the Enable Input node and when he exists overlap, use Disable Input.
Exactly like they do in the docs here: [https://docs.unrealengine.com/latest/INT/Gameplay/HowTo/ActorInput/Blueprints/index.html#enable/disableinputusage