You have to login as admin or moderator and then mark the threads you want to delete. After you mark them you go down to the page until you find a link for deleting the threads.

Hi Marco,

thanks for your answer. I thought there was a possibility in the old version of the forum where I could say, that all threads over (for example) 1000 should be marked all together. After marking them I could delete them. I can't find this function in the new forum. Seems I have to do it in your way...