If you received a Notice to File a Business Property Statement from the Assessor and your business is no longer in operation, you are still required to file the Business Property Statement. You should also include a note on the Business Property Statement indicating that the business has closed, so the Assessor does not continue to assess the property.
A limited-liability company ("LLC") is commonly referred to as a "hybrid" business structure, meaning that it shares characteristics of both a corporation and a sole proprietorship. It allows its owners to insulate themselves from personal liability for business debts, does not require the payment of separate business taxes (the tax "passes through" to its owners), allows for a great deal of flexibility with regards to its organizational structure, and is subject to fewer regulations and restrictions than are other types of business structures. Establishing such a company in Wisconsin is a rather straightforward process.
On the other hand, if you are not responsible for its tax, then you should declare the equipment in Part III of the Business Property Statement (Equipment belonging to others). Where equipment is declared in Part III of the Business Property Statement, the Assessor will also send a Notice to File to the person reported as the equipment's actual owner.