- create a page
- go to the edit -> access rights UI
- explicitely give delete right to a group or user
- go in the object editor, you'll see the XWikiRights object with allow and the "delete" right selected
- change the selection to give the "undelete" right and only this right (no delete right) and save
- open access rights editor again, delete right will still appear as checked.
Optionally, you can actually try to delete the page with a user from this group (or the user itself if the right was for a user) to see that it cannot actually delete it. Make sure however that it's not the author of the page, since author always has the right to delete.