I've removed my email address from the Mail app. How do I clear the storage still used by Mail app on my MacBook?

I no longer want to use the Mail app on my MacBook Pro. I've removed my email address from the Mail app, and removed the Mail app on my Mac from using iCloud. My goal is to free up storage space unnecessarily used - both on my MacBook Pro as well as in iCloud.

However, when I go to Apple > About this Mac > Storage > Manage - my MacBook Pro still indicates that Mail is using 374 MB of storage on my MacBook! Every site I've looked up says that I need to delete my emails in order to free up this space - WHAT??

How do I free up this space from my MacBook (without syncing back to Mail)? Hoping there's a straightforward, simple answer here. Thanks!