From what I remember, the way /r/place avoided sybil attacks was by limiting involvement in /r/place to anyone who owned an account before /r/place was created. There's no good analogue on the blockchain because addresses don't work like Reddit accounts.

An additional constraint was after changing a pixel you couldn't change another pixel for a period of 5 to 20 minutes.

The proposal is to use donuts for sybil resistance, which I think will work really well. This means there's a small cost to changing the pixel on the banner.

To add an additional constraint it might be nice to have something like the price of the pixel increases when it's bought and drops over time. This kind of simulates the 5 to 20 minute time out period to slow down the turn over of a particular pixel.

Another option would be requiring users to register their Reddit account with a particular address. That way we could actually prevent someone from placing 100 pixels really quickly back-to-back. This would allow us to force users to work together the way /r/place did. The down side is we'd be forcing users to give up additional privacy by registering their Reddit accounts with an address on the blockchain, which some people in our community won't like. So though I really like the idea of rate-limiting individual users to force people to work together, I don't know if there's a good solution to that in our case.