Bots
Bot protection
Bot protection here means making repeated automated form posts expensive.
It does not replace a network filter for large floods.
Scope
Where a proof of work check helps.
Background reading: What is a bot? and What is bot protection?.
Forms
Signups and comments
Each scripted attempt spends computing time in a Web Worker before your server accepts the post.
Higher cost
Limit
Not a flood filter
A volumetric flood is a different problem.
Keep your network protection for that traffic.
Application forms
People
No picture puzzle
Visitors are not asked to label images.
The check uses no cookies and no tracking.
Quiet for people
Add it to the form that gets spam
Start with the noisiest form and verify the token on the server.