When you have an employee whose office is a constant work in progress, the first point to consider is whether she is making any progress in her work. Although there are limits as to what is acceptable here, you should focus your attention at this point on her performance and interaction with the rest of the staff.
If her work is solid by all measures, then her actions need not be a major concern for you. There are some people who find that they work more productively and creatively when in a fresh environment, and perhaps she is one of them. She may also be looking for that perfect office layout, and when she finds it, her constant refurbishing will cease.
At the same time, if her focus on her office layout is so extreme that her work is falling behind, people who need her are unable to access her, or she is frequently distracted, then it is time to take additional action on this matter. Her actions could be reflecting any number of deeper problems, and they may actually be a cry for help.
You should meet with her and give her some feedback regarding the specifics of her work. Her reaction will help you design the next step.
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