Process

Sign in as you, then visit your blog. Little trash can icons should appear at the foot of each comment. You can delete them one by one, that way. Also, to forever rid your blog of ads, you might want to add a spam blocker. Blogger has one. All you have to do is go to the dashboard, and below the part that says "Settings" the spam blocker should be listing. Just turn it on.

Hope I helped!