How do you remove something from "External Libraries"?

I'm on a workstation that has Python27, but I'm trying to write in a virtualenv of 3.7. I can do this outside of PyCharm just fine by changing the PATH files, but IN PyCharm, I can't seem to exclude Python27 from the External Libraries.

I'm ususally writing python in a Maya environment, so this is somewhat new to me.

 What's the method for exclusion?