If you need to delete or select multiple posts in a single click from the wordpress dashboard itself here is the tip:
First Go to posts section in dashboard:
Select Screen Option >> and add number of post showing count to larger number. For eg : 250
Then 250 posts per page will be displayed. Then select multiple posts(All post check box on top of posts) and and move to trash.
If it shows any error, decrease the number of posts and try the same.
Its a tip not a official way. You can do the same using sql query in database.