* Seminar already implies a class or a series of lectures or discussions on a specific topic.
* Free indicates that there is no cost associated with attending.
Therefore, it's more accurate and concise to say:
* Free seminar
* Free class
* Seminar (if it's already understood that the seminar is free)
You could also use other terms depending on the context, such as:
* Free workshop
* Complimentary training
* Free webinar