You appear to be on the side that the user needs to go, based on your description of the situation. You've made a case here on why you think he needs to go. What are the counter arguments to keeping the user around? It seems that both users and administrators want the user to stay. As your site has an aspect of community moderatoration, this is even more striking against your point of view if others are not in agreement with you. Are you biased against this user for some reason? If so, that may be something you need to consider when you attempt to sway others toward your point of view. You mention that some user ignore the "damage" the user has done. Perhaps they see no damage. Something to consider as well: What you consider off topic may not be what others consider off topic. Are on and off topic topics clearly defined? If not, that needs to be discussed either by the administration team or by the community at large.
ProgramFOX makes a very good list of steps in his answer. The point I'd like to reiterate is that you need to communicate with your fellow administrators. But this needs to be done simply from a factual point of view. Leave phrases like "damaged the community" and "plays his games" out of the conversation. Instead, focus the discussion on facts.
You mention the usage of site analytics. If these truly show your point of view, utilize them. However, beware that statistics don't always show the whole picture. Could your site utilization have been lower because of summer vacation or a sports off-season or any other number of things? Think about this, because the other administrators will when you try to use statistics to convince them to remove this user. Could you have lost users for other reasons such as overly strict moderation or a limited topic scope that they feel they have nothing to contribute to any longer?
The discussion with the other administrators should also take the time for the opposing view to be shared as well. Some people want him to stay. Why? Or, even more importantly (more as a precedent setting measure for your community), is your user innocent until proven guilty? Does someone need to defend the user if your argument is not convincing?
If your administrators do come to your point of view and an action needs to be taken, take one. Decide on the action during your discussion. Then implement that decision. It's also important to ensure that your community policies and rules are updated to reflect the reason for the decision so that this type of behavior doesn't occur in the future. If it does reoccur, you can point to the rules and explain exactly what is being violated. These rule and policy changes need to be shared with the community.
In all of this, I'm guessing, your goal is some form of rehabilitation for the user. After the administration discussion occurs, some one from the administration team needs to talk with the user privately. Preferably this will be someone who was relatively neutral in the kick/keep debate. The discussion with the user should cover both sides of the arguments that were made and provide examples of behavior that people deemed inappropritate. "You did bad things", is not helpful. Instead, "In this post, you explicitly changed the topic of whale watching when a user mentioned his Grandmother had been on a trip and saw a whale once. You changed the topic to your grandmother's cookies and how they are the best thing since the sun started burning", is much clearer in what was done inappropriately.
Cover any disciplinary actions that were decided on as well. Explain why the action was taken. Even more importantly, discuss expectations when the user returns. Do you expect them to just lurk or are they allowed to participate again? If they can participate, if they don't hijack any more threads, is there a punishment if they contribute to a large number of topics in a meaningful way or should they limit themselves to only a few topics for the foreseeable future?