Have you already tried this (From the ePO 4.0.2 manual)
Deleting distributed repositories Use this task to delete HTTP, FTP, or UNC distributed repositories. Doing this removes them from the repository list and removes the distributed repository contents. Before you begin You must have appropriate permissions to perform this task. [LIST=3]
Go to Software | Distributed Repositories, then click Delete next to the desired repository.
On the Delete Repository dialog box, click OK.
NOTE: Deleting the repository does not delete the packages on the system hosting the repository.
I realise this post is slightly old, however I have come across the same problem and wanted to let others know how I got around the issue of wanting to delete a Super Agent distributed repository after the host has been taken off the network.
I think the OP is refering to Super Agents within the Distributed Repository list and not HTTP/UNC/FTP Repositories. The former does not display a "Delete" option next to them, whereas the later does.
I found that if I added a host to the Distributed Repository using the same hostname as the one I wanted to delete (add it as a HTTP, FTP or UNC it doesnt matter) and then delete it again then your original host will get deleted as well. This worked for me at least.