don't create a "deny" permission.... If you don't want a user or a group of users to delete something or access something leave it blank (don't add the user/user group or if you doin't want them to delete something... Leave the deny permission blank and also the permit permission blank and they wont be able to delete the file/foler!!!)... If you need more help on this, I can send you a screen-shot of how I have used the permissions and explain in more detail!!
Just e-mail me if you want the screen-shot!!!