My question involves landlord-tenant law in the State of: Wisconsin (Madison)

I am a landlord currently renting a six-bedroom house to five students. Recently laws have been passed allowing only five unrelated people to occupy a house. The students I have been leasing the property to have been there for two years prior to the start of this rental period (Aug 15 2009-Aug 14 2010), that is to say since 2007, prior to these laws being passed. One or two of the students have changed, but four of them have stayed on the lease the whole time.

I have recently become aware that although they only had five sign the lease, they have subletted the sixth bedroom to someone else. They have always been great tenants, but I don't want to be on the wrong side of the law here. What is the proper course of action? Can I serve them a letter saying that I have become aware that they have illegally subletted the sixth bedroom and that the sublessee must leave? How long must I give this illegal tenant to remove themselves and their property from the apartment. I have no qualms with the residents, just that they should not be subletting the sixth bedroom. I do not wish to bring them to court, I just want the house to be law-abiding and remove the excess roommate.