1. I highly doubt you can ruin your metabolism.
2. You can certainly slow it down by under eating chronically, however, even this fact is overblown around the web.
3. 15 calories per pound of bodyweight is the average for maintenance, meaning, energy in = energy out and you maintain your weight. If you were to eat that and feel very full or gain weight, chances are, you have slowed down your metabolism from under eating. If this is the case, and you are having trouble losing currently, I would suggest small, systematic increases in caloric intake back toward your natural maintenance. I have done this with clients with little to no fat gain. Other gain some fat during the process, but it is a necessary short term evil for a long term good.