Also, one point that I don't think was mentioned much.. Each dApp can have a way of listing the users that are in the 'trial' process so that the community can check them out and help onboard them with upvotes if they want. This helps outsource the job of filtering spammers out to the community, which is perhaps the most efficient way to do it. Ultimately there could even be some kind of onboarding bonus for users who 'curate' the best new users towards their full account.. But one step at a time ;)