Controversial “vigilante” anti-spam service Blue Frog has decided to shut up shop after a spammer threatened to mount more denial-of-service attacks on its servers and those of its partners.

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 denial-of-service attack.

In recent days, however, threats of more attacks – including on third-party and ISP sites associated with Blue Frog – have convinced the company running the service, Blue Security, that it 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 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 anti-spam activities and are exploring other, non spam-related avenues for our technological developments."

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 arm-locked 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 denial-of-service attack.

A number of large spammers complied, but one spammer – alleged to be PharmaMaster – appears to 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.