Ps - Now that I've deleted this user (who left our employ), their content is all owned by Guest which is a little screwy since we've lost the ability to track the owner of the content.
Sounds like this information is a little late for you, and as you already observed we do not have "disable" functionality.
Depending on your site you can do the following:
1. Change Password
2. Remove all permissions for the user (essentially making the person just a "registered user"
3. Ban the user.
I would recommend option 3. You can ban the user and disable their login, which is essentially what you are wanting to do.