What version of DP?
What is the OS-verion of the Cell Manager?
I have found that the easiest way to get rid of a client is to edit the 'cell_info' file, and remove the line that contains the hostname of the client you want to remove (make a copy of the file first). Without knowing what your cell maanger is, I can't give yo a express directory path, but, you should be able to do a Search for it. After deleting the line, save teh file
You may have at least one backup specification with this hostname in it. UNfortunately, there is no easy way to determine this, you just have to look at each backup specification, if you see an occurrance of this, then you can uncheck it and apply. I think that, given the error you are quoting " Connection to the remote client failed.Either the client is not accessible on the network or the inet process is not running on the remote client." that this is a good possibility
You may be able to do a Search of the Datalists directory (file system backups) or the Barlists directory (Applications Backups) to make it easier to find this hostname