How can I get rid of large tree tops?

You can burn them, but it's really dangerous. It can get out of control pretty fast.

UPDATE: Shears should now be used to get rid of leaves. They're 6 times faster than using your bare hands (and 4 times faster than using a sword).

Alternatively, you can use a sword if you don't really care about damaging it:

Leaves can be destroyed faster by using a sword, though swords lose double normal durability when used on leaves. As no other tool yields any increase in removal speed, leaves are best removed with bare hands, or a block or object unaffected by durability, so as not to damage your tools unnecessarily.

Finally, as Jonathan mentionned, the leaves will decay after a certain time if it's not attached to solid blocks (but I don't consider this a 'quick' way to get rid of them):

In Beta, leaves decay according to a simple formula: leaf blocks decay unless they are within 4 blocks of a log block, and connected to the log by other leaf blocks. Thus, harvesting the wood from a tree will cause the leaf blocks to vanish randomly one by one.

This is the OCD's best friend:

This is a Flint & Steel - you use it to light fires. In this context, you can light the leftover leaves on fire and they will quickly burn away. Be careful, though - fire "jumps" 1-3 spaces, so if you have closely-packed trees (or a wood house) they will also catch fire.

You have to get rid of all the wood, then the leaves will decay and disappear on their own. Stand where the original trunk of your tree was. Equip a piece of dirt (or some other solid block) in your hand. Then look straight down, jump, and right click to place the dirt. You will land on the new block of dirt. Repeat this, building a column of dirt, until you reach the remains of the tree above you. From there you can usually reach all the rest of the wood.

Also, start cultivating birch trees as soon as you can (they have the white bark with dark spots). According to this post on reddit, they never grow taller than six blocks of trunk, which you can harvest from the ground.