You can delete all the posts by:
1. Log into Blogger.com & select the blog
2. On the left, click Posts->All
3. Tick check mark in the box under New Post
4. Click Trash Can icon
5. Repeat step 3&4 until no more post
If you want to reset the theme as well, please follow the instructions below:
1. On the left, click Theme
2. Click Edit HTML
3. Click Revert widget themes to default->Tick a box in front of Select all widgets->click Revert selected widget
4. Click Save theme
5. Click Save arrangement