This was definitely one of the most annoying things about my new Drupal installation. In addition to having user signatures added to every post once it's been submit, it has also added a signature to a reply box BEFORE submitting, basically resulting in a double signature.
You can imagine how annoying this may be when people post a reply and every time they see a signature added to a FIRST line of their new post. To begin writing they have to either delete that pre-added signature or move cursor to the beginning of the line. It's a serious usability issue if you ask me.
So I asked a few times in IRC, searched the forums like crazy, but I found no solution except for a signature.module for 4.6. I am using 4.7 though.
So then I went to comment.module, searched for "signature" in a comment.module file and got only two occurences. The second one is the culprit. It is the 1324:
$edit['comment'] ? $edit['comment'] : $user->signature, '#required' => TRUE);
$form['comment_filter']['format'] = filter_form($edit['format']);
What I did to simply get rid of the added signature in a text area is delete this part of that line: ? $edit['comment'] : $user->signature
And it was gone. I tested to see if replying after this change works and it does so everything seems fine.
It is probably best to have the removed part only commented out as a slight reminder of where you made the edit. The final line should then look like this: