Thanks Dario and Stacy

I ended up revising the Blog_AddComment stored proc to allow Trackbacks and comments only if the AddedDate falls within last 1 month. For deleting trackbacks it was easy since all three fields Author, Website and Email are null (and I modified the comments on my site to require email address for every comment).

IF @Author is null  AND @Website is null AND @Email is null

BEGIN

SET @Approved=0

END

Of course, this solution is only specific to me, others may not find it useful. Regarding the "reply to" comments on my  site, I modified the source a little; I didn't find any suitable pre-baked modules around that I could use. Hopefully this will be incorporated in future versions  of DNN Blog itself.