Hi @Seungmin K
a) To cancel the account:
1. Sign in to your AdWords account.
2. Click the gear icon (Setting) and choose Account settings.
3. In the "Account cancellation" section at the bottom of the page, click Cancel this account. This option won't appear if your AdWords account is already canceled or if your user account doesn't have administrative access.
4. In the pop-up box that appears, you can follow links to troubleshoot common issues, leave us feedback about why you're canceling your account, and confirm that you want to cancel your account by clicking Cancel my account at the bottom.
To know more on Canceling Your AdWords account
b) To Pause The Campaign:
1. Select the Campaigns tab.
2. Select one or more campaigns to edit.
3. In the edit panel, select a new Status in the drop-down menu. When more than one campaign is selected, the new status applies to all selected campaigns.
Hope this will help.