Did you select the ones to be deleted while in the User CP screen? The check box for each that can be deleted is on the far right. Then, click the "Delete Selected' button.
If there is no check box when viewing the list in User CP, then it cannot be deleted. I have some that can;t be because they are in threads in the Uniden Archives.
That's because you are looking at the Manage Attachments that appears when you do a reply to a post.
If you go, instead, to the User CP (User Control Panel) as I've suggested, then look at Attachments there, you will see that selected attachments can be deleted.