Can I make an employee remove their excessive personal property?

I am an employer in California. During a recent walkouth of one of my office I noticed an employee with excessive personal items in their office. This includes, but is not limited to sheets dd on the wall as decoration, over 10 paintings, two leather chairs, a sofa, a round table and two matching chairs, and not to mention all of the knick nats. This is highly un professional and something has to be done. Are there any law that I can use as backup?