Turn off auto-tweet for an account
From within your Twitter account, go to Settings > Applications and revoke MailChimp's authorization.
Even if auto-tweet is enabled for a campaign, once you've revoked access at Twitter our call will fail and the campaign will not be auto-tweeted to Twitter.
Turn off auto-tweet for a single campaign
On the Setup step of the Campaign Builder, you have the option to tweet or not tweet a campaign upon sending. Just check or uncheck Auto-tweet campaign as.
If you are replicating a campaign, the auto-tweet setting is copied over to the new campaign like other campaigns options. You'll just need to uncheck the option before sending.
If you've already sent the campaign, you can try to revoke access as noted above quickly to stop it. If it's scheduled or a Draft, you can pause it and remove the option to tweet on send on the Setup step.