This one worked (if it does not: read on)

I tried some of these plugins because people try to make money from people using google fonts directly in Germany.
This one worked like a charm, I just had to get rid of one CSS-Import manually.

But there was one request to fonts.googleapis.com/css?family=Open+Sans I could not get rid of. It took me ages to realize, it was from the WordPress Admin Bar. So you need to check in a fresh brwoser-window, best a private one.

Thx to the people behind this plugin!