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A lot of users also post from mobile devices, which is usually behind a carrier-nat, which tends to swap the address very, very often

IP banning has lost a lot of its effectiveness when ISPs started putting people behind NATs and recycling their IPs (which was a long time ago), and further lost effectiveness with the advent of smartphones and mobile Internet connections.

Yeah, I stand corrected

Yeah and regardless of their efficiency, there are people who use dynamic IP. (not by choice)

Ip banning for new accounts together with creation time exceptions seems pretty good thought: almost all existing users would be unaffected. Only costs you growth untill you find some extras to put into the account creation process.

Which "extras", though?

You charge people 10bux. Something Awful is still chugging away on that model.

For a given value of Chugging Away, sure.