khmerox - you have 2 options:
1st - directly edit the template in use in components/com_comment/joscomment/templates/yourtemplate/index.html
2nd option is I guess recommended.
Go to to the backend - to the layout page and you should see some options to copy the template, to use the copied template and to modify it
Once it is copied, and you select the modify option you will get 2 new tabs - once for the css and one for the html. Edit the html and remove {poweredby} and you should be ready to go.
I have to write this in the documentation.