To stop filling or asking for saving on any site you may click extension popup in browser address bar and uncheck Enable auto-fill for filling password on that domain and Ask to save passwords to stop prompting you to save password.
Note that these options are relevant only to current active tab in the browser and works based on website’s domain level so disabling these option on tab with https://domain.com will disable it also for https://subdomain.domain.com
Just came to my mind that we can add this option to the saving popup, will discuss it with DEVs.