The thing is, the delete button brings up a little popup window with four options, each with a tickybox: 'delete comment,' 'ban [commenter] from commenting,' 'delete thread (all subcomments),' and 'delete all [commenter] comments in this post'. Underneath those tickyboxes is a button marked YES that you need to click to carry out whichever options you mark. I could click on the delete button itself with no problem, since it's in the blue header bar, but I couldn't do anything with the popup window -- I could not get at any of the tickyboxes, let alone the YES button. And tabbing and clicking is dangerous if you can't see what it is you're telling the program to do!

Since I found the workaround, I have also noticed that there is a separate delete-as-spam button in the header bar... but that also works by opening a little popup window, which would have opened under the embedded video and thus been useless to me. :-(

It's just bad design.