I did not receive a report from the User that first created the post so it may be that the issue has been resolved in a later version of CMS 1.8.
I remember there was a bug with an older version of 1.8 where display entries that had tags set could not be deleted, do you have any tags set for that Display Group? If so, can you remove them and then try to delete it again?
If this does not resolve the issue, I would recommend upgrading to 1.8.13 CMS to see if the issue is resolved. If you still cannot delete the Display Group, please let me know.