You can now cancel in bulk your inactive Google AdWords accounts from your manager account. All you need to do is login, select them and click edit and then cancel.
- Sign in to your AdWords manager account.
- From the page menu on the left, click Accounts, then click Management along the top.
- Check the box next to the accounts you want to cancel.
- Click Edit and select Cancel from the drop-down.
Here is a screen shot:
This should help those with lots of inactive accounts to clean things up quickly.