If you are the OP of the thread you go to the message that has been marked as the solution and then click on the three dots at the right hand side of the message an select the "unmark as accepted solution".
Keep in mind that a message can be marked as the solution although it might not be solution that you want ie in this thread there is no way for BTMail users to delete the blocked senders en masse.
As explained in the thread and as you have found out there may be a way for the BTMail email team to delete them in exceptional circumstances or there may be a way for some knowledgeable users to to that but it is not a route open to the vast majority of BTMail users.
