Scarring occurs when wire is wrapped too tightly or left too long and the tree starts to outgrow the wiring. A scar on a tree does not harm it and will not cause any issues in regards to the health of the tree. However, it is aesthetically unpleasing to the eye.
Although there is no real fast way to remove a scar, the best method is to let the bark layer over the wound and heal the cambian layer on its own. Some Bonsai trees especially deciduous trees and Chinese Elms, heal more quickly then others. Taking proper care of your tree and thereby promoting growth will help the tree heal over the scarring.