Showing it is making good on a promise six months ago to crack down on spam intended to take advantage of unsuspecting investors, the Securities and Exchange Commission on Thursday suspended trading in three stocks it said were “susceptible” to e-mail promotion schemes.The stocks, all listed on the Pink Sheets market, were Alliance Transcription Services, Prime Petroleum Group and T.W. Christian. Each displayed traits making them likely candidates for spam campaigns, says John Reed Stark, chief of the SEC’s Office of Internet Enforcement.Financial spam campaigns are classic pump-and-dump operations. The perpetrators buy a stock, spread false promotional information about it in unsolicited e-mails in an attempt to boost the price, then sell into any resulting rally.