You may want to manage which Leads and Contacts are included with the sync between your Salesforce instance and your Delivra account.
Only Leads/Contacts with the Delivra Opt In checkbox will be included. You can perform a manual uncheck of the Delivra Opt-In box for those Leads/Contacts that you do not want to be sent to Delivra. This must be done in Salesforce.
Here is a help center article that covers the process for mass updating the Delivra Opt in to be checked; tweak step 6 to use the appropriate value to deselect the Delivra Opt In checkbox: https://support.delivra.com/hc/en-us/articles/115000166627-How-do-I-update-the-Delivra-Opt-In-Box-to-be-checked-for-all-of-my-contacts-