Well, there’s also IP address. If you’re savvy you might be able to get around that as well, but it’s deliberately difficult to do sustainably for exactly that reason.
To my knowledge, there is no coordinated sharing of IP addresses between instances. Different instances are run by different people, so it’s very unlikely the IP address will matter.
A new username on a new instance is likely enough that no one will ever know, so long as your posts or comments don’t give you away.
Well, if you’re not a dick you theoretically shouldn’t get banned the first time.
If you are a dick, I imagine you’ll run out of instances pretty quickly, using that strategy. You could keep it up for a while, though. I do worry about the reckoning with spammers that will eventually have to happen on here.
There are already spammers all over Lemmy. There’s a coordinated effort to remove spam, it just doesn’t include IP addresses (which aren’t that helpful because they change and with CG-NAT entire neighbourhoods can share them, and with VPNs people not near each other can - plus if you have a dynamic IP then restarting your router can give you a new one).