Nelson, a junior who led the team in scoring and rebounding last year, missed a game in November under his first suspension.
Now he has been shelved for conduct unbecoming a member of the basketball team, Coach Ben Howland said.
"This continues a trend of very disappointing behavior by Reeves," Howland said in a statement. "I personally have worked hard with him to illustrate the importance of the code of conduct for our student-athletes, as well as the ramifications for violating it.
"I am disappointed that he has continued to fall short of my expectations."
Nelson averaged 13.9 points and 9.1 rebounds per game last year.
No timetable for a re-evaluation of Nelson was announced.