MovableType has commenting system, but it is not good. Most of time, thousand spam is sitting in the blog await the owner to delete them. If there are many thousands spams, it is hard to delete them in a short command. My experience is to use phpMyadmin to delete them. The sql script I use is: delete from mt_comment where comment_visible = 0. I used it to delete 63962 spams.
OK. I give up. I find Disqus. It is beautiful and easy to install.
First of all, I have to disable the internal commenting system of MovableType 5
Click Settings >> Feedback >> Comment Settings, uncheck the check mark box of Accept Comments.
Second, Go to disqus.com to sign up and add your site. When choose the code, please be noticed that MovableType plugins is only for MovableType 4, not 5. So, choose universal code.
Then copy the code in the first box and put in the very beginning of comments template. Save the template.
Keep the code already there, which will keep the old comments.
The last step is republish all entry archives.