I tried this:
pkill -KILL -u root
It turns out that wasn't what I wanted. Now I can't log in by SSH.
What is the correct way?
Edit - I've added the word "safely" to the question. I know that you should not kick out root users because they might be doing something important. But suppose that user is a customer service agent at a large hosting company who is charging you about $5 per minute as long as they are logged into your machine. Suppose you've told them the problem is solved and they can log out now and stop billing you, but they pretend not to get your emails because your boss already authorized two hours of work and they'd like to bill 100% of that. Suppose you aren't the average soulless corporate slave and you'd like to save your boss some money. So anyway, I kicked them out. I did it the wrong way, but no harm was done and I learned a few things, and I saved my boss at least $100.