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Comment Just take some computational resources (Score 1) 272

An easy way to get rid of automatic form spamming is to require some intrinsically difficult computation to be performed before submitting the form. This computation would be performed in a few seconds by Javascript while the user is filling the form. A robot would need to do the same computation, which spammers can't probably afford (unless if spamming from botnets).

Example of computation: the server picks a random string S and a random integer I between, say, 0 and 1000, sends S and md5(S+I) to the client and ask it to find I.

Numeric stability is probably not all that important when you're guessing.