In a CRM 2016 online real-time workflow, is it possible to avoid a record to be updated? In particular i created a real-time workflow of the type "before record status updated", and my objective is that I don't want an opportunity to be activated, if the value of a field of the opportunity is "yes". Is this behaviour achievable with a workflow, or I need a plugin?
Just for chucks and giggles: As you said it is a real time workflow, you could use a custom workflow activity to throw an error, that would prevent record creation or update as real time workflows are transactional with the create/update operation.
Recommended: Synchronous plugins were put in place exactly for this very reason, to perform business validations and complex business operations.
To use the system as intended, use a plugin.