LONDON—Three U.K.-listed small-cap oil companies are taking or threatening action against anonymous users of investor bulletin boards, the latest stage in a long-simmering row over whether these popular sites facilitate market abuse.

Empyrean Energy PLC, an explorer focused on the U.S., said Friday it is "investigating multiple potential cases of bulletin board participants posting possibly defamatory, misleading and false statements about the company and its management." It didn't say which boards it was looking at.

Its...