This could be done with something like hCaptcha or even letting the owners create a set of questions that are randomly given that the user has to answer. Ideally, this would be done in a way where the owner can write custom verification code, although I'm not sure how difficult it would be to make it flexible like that (I'm thinking along the lines of just having a script or something between the signup form and the actual part that adds the new account to the database, not some easy-to-use form builder thing).