If someone else is using our website

More than 20 people had started a registered nonprofit organization and was using a web site for different activities with general public. Eventually the organization become very popular. One of the member of the organization was responsible for the web site and the person had all rights for the domain. After 6 years the person suddenly started a new organization and changed all the contains of the web site without informing the old organization. My question is : What the old organization can do in this situation? The new organization is trying to still the brand/popularity of the old organization. Can we stop this web site?
Thanks,