Webauthn may make it easy because it supports platform authenticators like Touch ID / Face ID. Two good demos are at http://webauthn.me/ and https://webauthn.io/
It's now supported in all major browsers but platform authenticators are likely not supported on all OS yet.
I'm not sure it will be as secure as captchas though. Browsers will probably allow dummy platform authenticators for ease of development, which can be used by bots as well.
It's now supported in all major browsers but platform authenticators are likely not supported on all OS yet.