Matt Morris
The devil is going to be in the details on this, and there are probably dozens of follow-up questions that a nonprofit attorney (which I am) or an intellectual property attorney (which I am not) would ask you. But here are some thoughts.
This type of situation often arises when two or more groups or factions both claim to be the person or people who can act on behalf of the organization. Is that what is going on? If so, I would want to take a long look at the organization's bylaws and ask a lot of questions about what has happened to cause this person to go from being a board member sometime in the past to being a former board member now. And do they believe (or say) that they are still authorized to act on behalf of the organization as a board member? Or would they say that they are just pursuing a similar mission now that they have moved on. The approach to take would depend on some of these questions.
You say that the person is using the organization's name for personal use. In what way? The worst version of that would be somebody going out and soliciting charitable donations in the name of your organization and diverting the donations to their personal bank account. If that is what seems to be happening, the Missouri Attorney General's Office charity and nonprofit organization section is a good place to file a complaint. There is a link and a phone number on the AG's website.
Your issue about access to the domain name involves different questions. How did they do that? Access to the domain name is largely a matter of contract law between the domain registrar and whoever registered the domain name. Did the organization register the domain name, or did a well-intentioned supporter register the domain and now there's a dispute about should be allowed to use it? We would want to talk about the question above: is this a board faction dispute? Beyond the question of a faction issue, plenty of organizations (non-profit and for-profit alike) become victims of some form of domain hijacking. You have used the term "intellectual property" here, and if we're talking about the domain name it may or may not be intellectual property. And the extent of your intellectual property rights in that name will affect things like whether you've taken certain steps to protect that IP. We'd have to chat.
Finally, any other intellectual property? Are we talking about copyrighted materials that have been taken and are being used by somebody other than the copyright holder? Or are these trademarks that are being infringed? Depending on what type of IP and how it is potentially being infringed, next steps might include a "DMCA takedown" request or perhaps a lawsuit.