[robertlagrant]

Good idea! To prove you are a human, mine three Bitcoin.

[meowface]

Spam and DoS prevention were actually the original purpose of Proof of Work. Hashcash, the first (I think) anti-spam PoW algorithm, was what Bitcoin's PoW algorithm was inspired by.

[hsbauauvhabzb]

Why not something computationally expensive? Bcrypt this value 10000 times to continue

[sodality2]

Ideally you'd have something that's hard for the client but easy for the server to verify, though. so maybe prime factoring?

[tshaddox]

Or, the server provides a random string of bits with length n and requires a string of bits with length n+m that starts with the provided n bits and whose SHA-1 hash begins with 20 zero bits.

[wruza]

What nonceness!

[iszomer]

Imagine when a miner is used as Captcha and would only pass when a certain amount has been mined..

[kalleboo]

Spammers use botnets, so they have plenty of computation available

[immibis]

And if every site does this, they have plenty less. Imagine if you can only attempt your spam 20 times per second instead of 20000