Hide A Shipping Method

Hi, we offer free ground shipping on orders over $60. I've figured out how to add in the free shipping method no problem, but how do I turn off the default ground shipping method if the customer has qualified for free shipping?

In other words, there is no longer any point showing ground shipping costs if the customer has 'earned' it for free.

My ground shipping is determined by weight, so i've already tried adding a cost dependency; more than $60 = 0 - but this doesnt work.