I use the Goodbye Spammer plugin for MyBB.
First, install it after downloading the plugin form its homepage:
- Go to ACP > Plugins > Activate
- Go to ACP > Configuration > Goodbye Spammer Settings > Configure settings > Save
Then you can delete the posts/threads of a user as follows.
- Open the user/spammer editing page as an Administrator or moderator or users in a group allowed in the option of Goodby Spammer plugin.
- In the "Moderator Options" tab, click the "Use the Goodbye Spammer tool on this user" link.
- Then you can ban and user and delete the posts/threads created by the user. Be very careful here since it may deletes many posts/threads.