Preventing someone from creating thousands of users
How are they creating this many users so fast and how can I prevent it?
You should think about:

Account activation with email
Or prevent too many api cals from one ip to your register route (maybe someone is just destroying your app with bot)
I made a simple Flask app that has no sort of auth since it doesn't really require it. Today my database reached 100,000 users when it normally gets about 5 in a day. I looked at my analytics but saw no one has been on the site in the past hour. 
How are they creating this many users so fast and how can I prevent it? I had to take my database offline since it's been happening all day.