i mean the comment itself is saved as a post (status message).

check this site http://ufancha.co.cc/elgg/  and you will understand what i mean.

there are comments to a post "this is a comment" and "second comment". they are under the post and independently as a post.

Ah, you're using the river as like a facebook wall.  In that case you will want to to disable river creation for comments.  I don't remember the hook offhand, maybe 'create', 'river' (at a guess) but you can hook into that, check if the subject is a comment, and return FALSE if it is.

Look in the docs for how plugin hooks work: http://docs.elgg.org/wiki/Plugin_Hooks