This feature and its page has not changed since at least 2007, probably even further back. I compared UBB.threads 7.5.9 to as far back as version 7.2.1.
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/admin/deleteuser.php?uid=999
The options for deleting someone in 7.5.9 who has posts, are:
Warning: Deleting a member can not be undone.
[*] Do not delete this member. [*] Delete member and do not alter posts. Ownership will be set to Anonymous with an Anonymous name of: Gremelin. [*] Delete member and delete all posts. If there are any replies to a post by this member, they will be attached to the post's parent. If the user's post is a topic, the entire topic will be deleted.
