To uncast anything is merely to reverse the process, such as when a witch casts a circle she will reverse the process she used initially to uncast it.
U is also for unity, something I would love to see a lot more of. A lot of witches get on just fine, recognise that we all have different views on stuff and are all entitled to our own opinions on something. But there is, as with all of humankind, a small group who will verbally attack anyone who disagrees with their own way of thinking. Remember my post on the fluffy bashers? To unite as one group, i.e. witches/pagans etc with similar views about such matters would be far better than attacking everyone who holds a different opinion to us.