Israeli firm Blue Security's controversial vigilante antispam service, Blue Frog, has been closed down after a spammer threatened to mount more DoS (denial-of-service) attacks on its servers.

Two weeks ago, an unknown number of users of the service were bulk-emailed by a single spammer telling them to stop using it or risk receiving even more spam. Its websites were also rendered unreachable on 2 May, most likely by a DoS attack.

In recent days, however, threats of more attacks – including on third-party and ISP sites associated with Blue Frog – have convinced the company that the service has no future.

"We cannot take the responsibility for an ever-escalating cyberwar through our continued operations," Eran Reshef, CEO and founder of Blue Security, was quoted as saying to the Symantec-owed news outlet SecurityFocus.

"As we cannot build the Blue Security business on the foundation we originally envisioned, we are discontinuing all of our antispam activities and are exploring other, non-spam-related avenues for our technological developments," Reshef said.

Launched last July, the 'do-not-intrude registry' service has always been as controversial as it was innovative. Anyone signing up to Blue Frog was added to a mailing list that spammers were pressured into agreeing not to use. If they sent spam anyway, they risked being inundated with automated email returns, which critics pointed out was a form of DoS attack.

A number of large spammers complied, but one spammer – alleged to be PharmaMaster – appears to have decided to strike back, fearing the service's success could set an example to others. It is said to have signed up 450,000 email users at its peak.

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