This isn’t to defend them - it seems very odd to immediately permanently suspend accounts like that!
Lots of people use vpns for work or privacy, I think that method would have a lot of false positives. In cases where trial account usage costs you very little or nothing I'd err on the side of not blocking users.
What makes you think that? This is you saying that that detection service is not very good, FWIW.
While certainly true, in my experience unknown VPN IP addresses can be even more problematic since for them, even providers willing to support the interests of VPN users just see lot's of suspicious usage from you IP address without any real explanation.
> This isn’t to defend them - it seems very odd to immediately permanently suspend accounts like that!
Normally I would agree, but since the account is not only just a few days old but also didn't post or create comments, I think that immediately banning it makes sense.
For a new account which didn't create any content yet the user isn't loosing anything important if the account is deleted and since the user is unlikely to have any real attachment to the account yet, they are more likely to just create a new account than to deal with any warning/suspension/whatever procedures.
IMHO you shouldn't expect your account to stick around until you have at least some non trivial content in the account.
Of course the main account is a different story.