Remove file attachment to post added by custom meta box

Hi. I’ve added to my template custom meta box to attach file to certain posts.

I’ve done it this way: http://code.tutsplus.com/articles/attaching-files-to-your-posts-using-wordpress-custom-meta-boxes-part-1–wp-22291

Now I don’t know how to remove attached file.
Does anyone know what’s the best way to do that?