You can suspend any account under your Reseller Account in WHM at any time.  To do this you will first need to log in to WHM.

After this you can search for Manage Account Suspension and select the option in WHM.

On the Manage Account Suspension screen you will be able to to see all of your accounts listed.  Here you'll be able select and account, enter a reason for suspenion, and then suspend the account.