Christopher Aber was ordered on Monday to stay at least 100 yards away from Rooney and his Los Angeles area home. Aber is the son of Rooney's eighth wife January.
A local lawyer was given temporary conservatorship powers over Rooney and his estate, at the actor's request, in a ruling by a Los Angeles Superior Court judge.
According to court papers, Aber is a regular but unannounced visitor to Rooney's home, and the diminutive actor who was once a close friend and co-star of Judy Garland in the Andy Hardy movies hides in his room to avoid his step son.
"He threatens, intimidates, bullies and harasses Mickey," according to papers in support of the stay-away order. "Mickey is effectively a prisoner in his home."
"Chris has blocked Mickey's access to his mail and will not provide Mickey with any information about his finances, other than to tell him that Mickey is broke."
The documents also allege that Aber has deprived Rooney of medications, food and has confiscated his passport and other identification cards.
"Mickey is extremely fearful that Chris will become physically threatening against Mickey and may even attempt to kidnap Mickey from his home," the court papers add.
Rooney rose to fame as a child star in the 1930s and 1940s when he made more than a dozen Andy Hardy movies. He appeared frequently alongside Garland and in his heyday was one of Hollywood's biggest stars, receiving a junior Oscar in 1938.
Rooney continued to work in movies and television into his late 80s, appearing in the 2006 film comedy "Night at the Museum," among other work.
Christopher Aber is an actor who had bit parts in TV shows in the 1990s. He is one of two stepsons of Rooney through his 1978 marriage to singer Jan Chamberlin.
Aber was also ordered to stay away from his brother, Mark Aber, and his former sister-in-law, who both live at Rooney's house.