Once the emails are processing to be sent,  you cannot stop the emails. You can always pause your campaign before the emails are processed to be sent as described here; How do I pause a Campaign. 

If you really need it to stop. You can disconnect the mailbox by following the steps here. You may also contact [email protected] for us to temporarily disconnect your mailbox in case you don't have access to your account. However, we don't really recommend this alternative for once we  disconnect your mailbox. the reply detection will no longer work and any emails connected to this email will not send. That said, you can also change the sending limit to "0" as an alternative.