If you do have permission to permanetly delete posts: Either click the checkbox on the lower-left corner and a dialogue will come up, look for a dropdown, find "Delete Posts" and it will automatically bring up yet, another dialogue, click on the checkbox with "Permanently delete Selecting this option will permanently and irreversibly delete the item."
Or you can just go to the post itself - find the "delete" button, and you're presented with two options - click the "Permanently delete Selecting this option will permanently and irreversibly delete the item." option.