Google LLC, Yahoo! Inc., and Microsoft Corp. cemented their win against claims they conspired to “flood” online searches with “scam locksmiths” so that legitimate ones would have to pay for sponsored results.

The Communications Decency Act bars the lawsuit by 14 locksmith companies, the U.S. Court of Appeals for the D.C. Circuit ruled June 7. The law broadly shields “online intermediates” from liability for republishing third-party content on their websites,

A federal judge initially tossed the case in...