Can I disable the sync for a specific audience?

We connect Salesforce to Mailchimp at the account level. All the audiences under that account will be synced to Salesforce. Therefore, you can't disable the sync at the audience level.

We recommend creating multiple accounts if you need to limit what syncs to Salesforce. Mailchimp support may be able to help you set up subaccounts, if your type of Mailchimp account allows it.