You can either delete the issues that you have read or the entire application.
To delete only issues please follow this steps:
1. In the Library tab, go to the top of the screen and tap "On device"
2. In the top right you will see "Edit" tap it.
3. Here you need to select the issues that you wish to delete (only tap once in each cover)
4. Go to the delete icon located in the top left of the screen and tap it.
5. It will pop a small window, tap delete to confirm.
To remove the entire application:
1. Locate the app icon in your Newsstand folder.
2. Tap and hold the app icon until it wobbles.
3. Tap the X that appears above any of the applications you wish to delete.
4. Tap Delete to confirm.
If you change your mind in the future and would like to read the magazine on your iPad again, you can install the app again through the App Store.