OK, aside from the fact that these 'weird occurances' sound like miedra del toro, pentagrams (five pointed stars encircled) are not evil symbols. From the perspective of Paganism, a pentagram is (as others have pointed out) just a symbol. They have no power, in and of themselves. Those who use them come in two varieties: Wiccans and other Pagans, whose central philosophy is often entirely harmless; and Satanists, whose rituals couldn't summon a bad odor if they had all been eating beans and cabbage.
Suggestion 1: Forget about them. They're just defacement of property. Resand your floors, or leave the scratches under the carpet. They have nothing to do with these 'strange occurances', I guarantee it.
Suggestion 2: Visit your local bookshop and pick up something by Ray Buckland or Raven SilverWolf or something similar in that area of your bookstore. Look for some kind of cleansing ceremony or ritual. Burn sage and cedar, and waft the smoke around the pentacles, clockwise. Say prayers, whatever. It won't do anything more or less than the pentacles are doing, but it might make you (and him) feel better.
Suggestion 3: Join the 21st Century with the rest of us. Grow up and stop playing at spooks.