It would be nice if you could delete hosts even if they achieved credits.
I suddenly switched one setup and the old one disturbes me as the host has the same name as the new one and when I reattach him to Seti Boinc I dont want to see two of them in there

Let me see if I have this straight. You have an old computer, and a new one the new now has the old ones name, and the old one has a new name? If you have not actually detached, crank up BOINC on the old computer and the name on the web site should change (it has for me). If you have detached with the old computer, you can attach and merge.

The reason that a computer cannot be deleted if work has been sent to that computer has to do with the tracking of which work went to which client. The servers need to track this to make certain that only the computer that was sent the work returns the work. All results must have a computer attached, and you don't really want to delete the results.

What they could do is to allow you to hide the zombie computers.