On a network started by 2-3-10 people, the first new members would need to be vouched by a percentage of those to get in - and so on.
If someone down the line does some BS activity, the accounts that vouched for it have their reputation on the line.
A whole tree of the person who did the BS and 1-2 layers of vouching above gets put on check, gets big red warning label in their UI presence (e.g. under their avatar/name), and loses privileges. It could even just get immediately deleted.
And since I said "identity based", you would need to provide to real world id to get in, on top of others vouching for you. It can be made so you wouldn't be able to get a fake account any easier than you can get a fake passport.