Since v.4.0.0.504 there are some updates on the comments deletion in the blog/podcast.

Here are the rules about who and how can delete comments:

1. Only comments with no replies (last level comments) can be deleted;
2. Admin(s) and users with "Edit" access can delete all comments (when rule #1 is met);
3. Regular (registered) users with "view" access can delete only own comments (when rule #1 is met);
4. Users with "Edit own posts" access can delete own posts comments by rule #2 and others' posts comments by rule #3 (when rule #1 is met);
5. Guests cannot delete comments;