Okay.... So there is this thing about werewolves where you have to cut the head off and burn the head after killing the werewolf cause if you don't then it will rise from the grave 3 days later as a Vampire.

Then why did The Captain stay dead after Seras killed him? Yes, I know there is the fact that the creators of the show might have not known about this fact, or simply decided to not apply it, but there is also the chance that what I previously just stated isn't the reason. What if Seras was just that powerful, so she could overcome that rule?

Or, the more depressing reason.... The Captain didn't want to come back. He did smile when Seras delt the final blow, killing him. Vampires and werewolves wouldn't be there if it weren't for humans...  Humans have determination.....He simple didn't have the determination to live..... Or he used his determination.... To stay dead....