The Lounge is rated Safe For Work. If you're about to post something inappropriate for a shared office environment, then don't post it. No ads, no abuse, and no programming questions. Trolling, (political, climate, religious or whatever) will result in your account being removed.
Just use the good old send some random words (or numbers) to the user's email account.
If that is not secure enough, then make sure the user apply with 2 email accounts, so that your service can send two sets of different random words (or numbers) to the user's two email accounts. Which may be as safe as a "2 factor authentication services". No fumbling with phone SMS / swipe here / swipe there, etc, etc.
And if that is not secure enough, then make sure the user apply with 3 emai ... ... ... ...
This makes me think of an old song by Tool, "Disgustipated." During a spoken interlude they say,
And I begged, "Angel of the Lord, what are these tortured screams?"
And the angel said unto me, "These are the cries of the carrots, the cries of the carrots!
You see, Reverend Maynard, tomorrow is harvest day and to them it is the holocaust."
And I sprang from my slumber drenched in sweat like the tears of one million terrified brothers and roared, "Hear me now, I have seen the light! They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers!"
Can I get an amen? Can I get a hallelujah? Thank you Jesus