It looks like it deleted the posts but not the topic, which happened sometimes in 7.0.x. You can delete it by running the following SQL commands in the control panel. First:
select TOPIC_ID from ubbt_TOPICS where TOPIC_SUBJECT='Welcome to the Forum!' and FORUM_ID='8'
That will return the actual topic #, say 32 for example. Once you have that you can delete it with this, substituting 32 with the actual topic id:
delete from ubbt_TOPICS where TOPIC_ID='32'