One possibility is that you have 100 addresses already blocked and thus Yahoo mail can't delete that particular mail and add the sender to the blocked list. Theres 2 ways to overcome it, if thats the problem. 1) To remove some blocked addresses from the list of blocked addresses and 2) to upgrade to that Yahoo Plus!.
The 1st option is definetly the cheaper of the two.
Admit nothing, deny everything and make counter-accusations