Gosh, Deborah, I hate to hear that your employees are so frightened by this.
Do you know the history behind the building and/or land your shop sits on? Sometimes just knowing what you are dealing with makes it a little easier. I'd rather be haunted by the ghost of a kindly little old lady vs. a mass murderer any day!
If a light touch is all that has been experienced, maybe this is a protective spirit, reassuring you that he or she is glad you are around and just acknowledging your presence. You may want to try explaining that to your employees.
Something that I have found helpful is to actually say out loud sometime when the shop is empty, "Hey, I know you are here. You are welcome to stay, but please don't frighten my employees or customers..." In other words, address this spirit as you would a living person.
You might be surprised at how cooperative your ghost might be!