YOu tell him "I know all about you and ______, and it needs to stop -- now. It's incredibly disrespectful to me, and to our marriage. Even if you are divorcing me, it's still disrespectful and I won't tolerate it in my own house. I suggest you find another place to live; I'd like you out by the end of the month."
Who pays for the internet connection? If it's you, stop paying for it. If it's him, then set up a jammer that won't allow him access. You shouldn't have to put up with this kind of disrespectful, crap behavior, in your own home.
So what is the "or else" consequences? You know him best, and can come up with your own list, but I'd start by exposing to his parents, if they're still living, and with the other woman's husband, if she's married. That usually puts a stop to it right there.
Yes, he's already filed for divorce, Waterbur, but you still have your self-respect.
