John Ablewhite of Manchester, Kerry Whitburn of Birmingham, and John Smith of Wolverhampton admitted that they dug up the body of 82-year-old Gladys Hammond from the graveyard at a country church, The Telegraph reported. She was the mother-in-law of one of the Hall brothers, who bred guinea pigs for medical research at Darley Oaks Farm in Staffordshire.
Judge Michael Pert called the grave robbery "the lowest point" of the group's campaign against the Halls, which also included the theft of hundreds of guinea pigs.
Although the Halls abandoned raising guinea pigs, the body was only returned after the suspects' arrest, when Smith led police to it.
"Having stolen the body you used it as a weapon," said the judge. "We are not going to start guarding country graveyards on the off chance that some other lunatic fringe group emulate you."
Another member of the group, Josephine Mayo of Birmingham, received a four-year sentence.