Well, if you don't mind a bit of philosophy, true evil can never be destroyed only in the sense that true evil does not exist. Evil is darkness compared to light (an analogy made explicit in Star Wars, of course) - and darkness is not a property by itself, it is simply the absence of a property. Even the Emperor had good in him - he was alive, and that's a "good" property
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But in regards to force ghosts, I don't think the film gave us any reason to believe that Kylo Ren is seeing any ghosts at all. There's no reason to think that his little Vader shrine is anything more than just a mute idol. He might look to it for inspiration in the same way that you might go for a walk in the park for inspiration, but that doesn't mean that Vader speaks to him any more than trees speak to you.
Oh, and as for Episodes 8 and 9. After much careful deliberation and a few very interesting conversations about the patterns visible in the original six films and how these patterns appear to be being repeated in the new films, I'm going to make two predictions about them. We'll have to wait and see how utterly wrong I will turn out to be
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1. Rey is not related to Luke or Kylo Ren. She is not a lost sister or lost daughter. Her origins are similar to Anakin's. She will romance Luke, and bear a child, most likely a girl (though possibly twins again, since it runs in the family). In a Dune-like twist, this is what Snoke actually wants - he's trying to breed a superior Kwisatz... err, Sith. In this sense, regardless of who he is, he is actually continuing the work of Darth Plagueis. In the same way that it was implied the latter was responsible for Anakin's birth, we may well find out that Snoke is responsible for Rey's.
2. Rey will confront Kylo Ren and Snoke in a scene reminiscent of the Anakin-Dooku-Palpatine scene in Episode 3, and of course, the analogous Luke-Vader-Palpatine scene in Episode 6. Continuing the Star Wars tradition of scenes that repeat past scenes with a variation, this scene will resemble both of these, but will unfold differently. Just like in Episode 6, Snoke will invite Rey to join the dark side. The twist is that Kylo, feeling guilty about his father, "succumbs" to the light side when he realises that Snoke actually means Rey will replace him, and understands that he's been duped and manipulated all along. In the same way that Luke tried to attack Palpatine and was deflected by Vader, we will see Snoke attacked by Kylo, and deflected by Rey, who then proceeds to kill him and replace him. This, of course, will pave the way for a second trilogy, where Rey's daughter squares off against Rey to bring her back. I do not, however, have any idea about how they will make this different to Episodes 4-6, other than the fact that this time it will be a "girl thing".
...And no, I am neither drunk nor smoking anything
. The main problem with the above is the presumption that the new directors and producers will be interested in continuing the pattern that Lucas had established (with total authorial control) in the first six episodes. This is certainly not a given. I think one of the main questions with the new Star Wars films is the question of authorial control - are these films going to ultimately have one author (Kathleen Kennedy), or do the film directors really get sufficient freedom to exert authorial control over their individual films.
