GEOCODING - How to protect my KEY?

[Magnet Comunicações]

If Geocoding API cannot be used when a key has a referer restriction (URL) then how can I ensure that other people will not use my key and I will have to pay for it?

[George (Cloud Platform Support)]

You may restrict your API key in two ways: application restriction or API restriction. This is detailed on the "Get an API Key" page. Moreover, Premium Plan customers may choose to authenticate requests using an API key or a client ID and digital signature. 

You have published your message on Google App Engine Group. This is a Google Maps Platform issue; you should publish in the appropriate Group, and consult Google Maps Platform Support.