Hi Somen
Try this...
Go to the Blog page type in the tree, under the form tab untick the 'Allow comments from anonymous readers:' and tick the 'Require e-mail address from readers:'. This should help.
Along with this, make sure the Comments View web part has 'check permissions' selected, which would make sure only selected people/roles can see the comments view/add comments, which you may want to configure.
Another thing that could help is making sure the 'Comments must be approved:' is ticked to help moderating comments.
Thanks
Ilesh