MLB executive vice president Rob Manfred notified teams of the action Tuesday in a memorandum to club presidents, chief executive officers and general managers.
Nunez, a consultant for Cabrera's agents at ACES Inc., took responsibility for fabricating a website that the San Francisco Giants All-Star outfielder was relying on as he prepared for a grievance to challenge a drug test that was positive for testosterone.
Cabrera dropped his grievance last week after MLB uncovered the scheme. He was suspended for 50 games.
"Please be advised that commissioner Selig has directed that all major league clubs are prohibited from granting Juan Carlos Nunez access to their clubhouses or other nonpublic areas," Manfred wrote in the memo, which was obtained by The Associated Press. "Nunez is affiliated with ACES Inc. sports agency. Nunez is currently under investigation for misconduct related to our recent matter under the joint drug program.
"In addition, Nunez is not certified as a player agent by the Major League Baseball Players Association. Clubs should not conduct contract negotiations with Nunez or otherwise deal with him regarding players on the 40-man roster."
Seth Levinson, who along with his brother Sam runs ACES, said Nunez was a paid consultant for the company.
The memo was first reported by ESPN.com.