Are you trying to remove the comment from the manage tab or the feedback tab? I think the manage tab comment delete might not be working, I noticed that this weekend.
You should be able to delete the comment from the feedback tab no problem.
If you have access to your database via phpMyAdmin, go to the plogger_comments table and delete the comments directly from the database. You can change the default number of rows displayed from 30 to maybe 100 or so, and it may not take quite so long to delete them that way.
Do you have any idea how many legitimate comments you have in your gallery?
You also have the option of placing your comments in moderation, which means that you have to approve them before they show up in your gallery. It won't stop the comment from being input like the CAPTCHA will, but at least any that do get through won't be visible publicly.