The first two times Rand killed him, he was back in the dreams a couple of chapters into the next book. I think that whoever's idea it was about the black cord being necessary for ressurection is right. The first time Rand "killed" him, he cut the blackness (the connection to saidin, I believe, and then shot him with a lightning bolt, and burned him up. The next time, he stabbed him. The third time, he shot him with lightning, and stabbed him. However, the third time, he cut the connection to the Dark One, as well. Assuming Ishamael isn't Moridin, or Shaidar Haran (like Tamyrlin said to Mosleth, it would bug me), then he's gone for good.