The best way is to use flint and steel if you don't care about the wood.
Otherwise, you will want to create a roof limiting the blocks the trees have to grow because you won't be able to remove all the wood and the leaves will remain.
Of course, you could use the traditional method of creating a 1x1 tower of a common material to reach the wood and then destroy it afterwards.
But hey, isn't mindless destruction of blocky virtual trees fun?