You can delete styleguides under your workspace from the Styleguides tab. Hover over a styleguide and select “Delete styleguide” under the settings menu.

Type the styleguide name you want to delete and click on the “Delete” button:

Once a styleguide that is linked to a project is deleted, the colors, text styles, and components inherited from that styleguide will be removed from the project. If it's a child styleguide though, you'll keep seeing the parent styleguide data.