Can we discipline an employee for the actions of his significant other?

Q: We have an employee whose girlfriend has come in and wandered through the production floor without permission. We have escorted her out of the building twice in the last month. We also talked with the employee about his girlfriend. What are our options if she comes in again? Should we call the police? Is there any way we can discipline the employee because his girlfriend keeps calling in several times a day wanting to talk with him? – Carol, Washington