i want to delete all of messages one member

For a start to prevent this happening again, make sure that guest posting is not allowed.

Also, make sure that the flood control in the admin section is enabled and increase the time span between posts and how many posts can be made each hour (flood control doesn't work if guest posting is enabled)

To delete the posts you maybe be able to use the batch delete tool in the admin area and delete posts by date, but this may also delete other posts as well, but maybe the best option, unless you want to get your hands dirty and edit the database directly.