Turning Off Commenting For New Posts
Settings > Discussion > uncheck "Allow people to post comments on new articles"
To turn off commenting for already publish posts
Posts > select them all and "Edit" under bulk actions and hit apply > choose "do not allow" next to comments and hit update posts.
Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.