If you want to delete a bunch of posts, the easiest way is to filter by category.
First, go to Posts and filter the category by selecting the correct category in the category drop down. Next, click the checkbox next to title so you can select everything in that category. The Bulk Actions drop down will let you move a ton of posts to the trash at once.
This is all even easier if you go to Screen Options and change the number of Posts to display to a high number.