ZynSync’s calendar sync does not delete events for email by design. This was implemented initially since many customers schedule events in email to be copied into Salesforce. An organizational setting is under development to allow all users from a specific company/organization to opt to have events deleted from Outlook or GMail. If this configuration is desired, please notify email@example.com and the change will be made when available.
The org-by-org setting to enable deletes is due to release in mid 2018.