Workspace Rule to prevent Save

Summary

I want to prevent user from selecting a certain product

Content

So in our system we have some products that have subproducts  underneath.   In these cases, we want our agent to select one of the subproduct, not the main product.

I thought I could accomplish this as a workspace rule.

I can certainly figure how to send them a message when they select the main product.  But it is only a warning.   They can go past and still save the incident.

Is there a way for me to prevent them from saving the incident with that product selected?