Author pages indexing

Hi there,

We want the author pages to be excluded from indexing in the search engines.
We use Yoast Seo plugin, unfortunately there is no option there to specifically exclude the _author pages.
Only:
post types:
Avia Framework (avia_framework_post)
Portfolio Items (portfolio)

And taxonomies:
ategories (category)
Tags (post_tag)
Format (post_format)
Portfolio Categories (portfolio_entries)

Any ideas?