I think this is the sort of thing where the disagreement between madchef and Neriel started. It's definitely possible to stop slut level from rising. I have a save on day 87 with 58 SL, last passive gained on day 71. She hasn't even kissed anyone yet and boss of level 4 is down. The reward for this achievement is the accomplishment. Yes, the game is designed to slut Karryn up. Thus, the challenge of keeping her as pure as possible. There will undoubtedly be an ending that acknowledges this accomplishment based on the letters you receive from keeping low SL after each boss, and considering there was a pure ending in Melty's Castle.