How to delete all Facebook wall posts from one person?

I want to delete the wall posts that I left on their wall and I want to delete all the wall posts they left on my wall. How do you do that? I tried to delete some of the wall posts I left on their wall, but when I see the Facebook friendship we have, some of the wall posts are not on even their profile... I can't delete them from the friendship page which is really stupid. I don't want to delete or block the person... I just want to get rid of all the wall posts we left on each other's walls. Please help.