Need to hide Shipping Details when a customer chooses pickup as a delivery option

[mariagracia]

How come customers can still able to enter their shipping details even when they choose "pick up" as a delivery method? They deliberately do this to avoid paying shipping fees. This gives us a headache every time we need to fulfil their orders and we always need to send them another invoice for freight. What should we do to prevent this from always happening? Thanks in advance.