Can I delete all the entries in my journal?

Not automatically. You can delete all imported entries, but you can't automatically delete all the entries in your account. If you want to delete all the entries you posted to Dreamwidth itself (not just imported entries), but you don't want to delete your whole account, you'll need to delete each entry individually.

You can't delete all your entries automatically at once for security: if someone gains access to your account without your permission, they would be able to permanently destroy your entries very easily. It also prevents accidents.

If you have a paid account, you can use the Mass Privacy Edit tool to make all your entries private instead of deleting them.