Can we turn off Storyline's "corner radius" feature when necessary?

That's not possible at this time. You can submit a feature request here: https://www.articulate.com/product-feature-request/

• Submitting a request doesn't guarantee that a feature will be implemented. But the number of people who request a feature might have some impact on the decision-making process.

Hello Saumitrajoshi, 

Thanks for reaching out! As JudyNollet confirmed (Thanks so much, Judy!) , this is not possible right now. I've shared your insight with our product team. We'll be sure to share any future updates in this thread so all are in the loop!

It is indeed a very good question, sometimes it can happen that I make a box with rounded corners when I just wanted to enlarge the box.