well if you dont want session from old servers in your idb (if you dont want to restore anyhing from it) you could export the idb (omnidbutil -writedb command). after that, navigate to the files generated. you should finde a file named "hosts" where all the host from the cell are listed. delete the host which you dont want in your idb anymore. safe the file and do an import (omnidbutil -readdb). after that, you should not find sessions from those hosts and you wont be able to restore anyhting from those hosts.
before you do this, make sure no backup/restore is running, stop dp services with omnisv -stop and copy the whole omniback folder as a backup in case anything went wrong....