There is a flag that controls the clients and contacts that sync through to workbooks from Central. This flag is set in an extra field.
From a Client/Contact record, click the Extra tab and locate the extra field 'Exclude from Sync'.

If the value is set to 'Yes', this will not sync the record with Workbooks. If the value is set to 'No', this will sync the record and and subsequent changes to the record to Workbooks.