How to kill a pyracantha!
 

I have had an apple tree that has been growing nicely from seed on the border of my garden. Its 10 years old and is only a meter high. It hasnt been looked after very well but I am in the process of regenerating it to its full potential. However my rude neighbour has planted a ruddy pyracantha right next to it!

Has anyone got any idea how to kill this thing off. Im very worried about my apple tree. I want to be able to kill this pyracantha without my neighbor noticing . Me and my neighbor do not get on so simply asking him to move it is out of the question.

Any ideas please!
Thanks,
Liam