Simon, editing the content of somebody else's post is somewhere between wrong and illegal.
You can always delete the post entirely, or ban the user, but do not edit the content. I live in a former dictatorship country, and I mean it when I say it was wrong what you just did. The fact that is technically possible to edit, doesn't mean you should do that. I did forum moderation for many years, and seen a lot of crap, from spam to personal insults.
Anyway, editing is both wrong and time consuming, it is like you would lie about what somebody else said. Just delete the entire post and move on. Nobody will give you back that time spent editing.