I solved this (and a whole host of other customization problems) by replacing the bbp_forum_freshness_link() function call in loop-single-forum.php with my own function, which I copied from bbp_get_forum_freshness_link() and then edited.
In there, I changed a call from bbp_get_forum_last_reply_title() to bbp_get_forum_last_topic_title(), since the only difference I could see between the two was that bbp_get_forum_last_reply_title() adds the cluttered “Reply To: ” to the string.
You can probably achieve this with filters, as well.
