In a statement, the Hall of Fame coach said he hasn't yet determined a punishment for Hairston, who has also been cited twice by police while driving a rental vehicle linked to a convicted felon.
"P.J. and I have had several discussions already and he knows he has made serious mistakes and there will be serious consequences as a result," Williams said.
He said there are "several options available" as punishment, including a possible suspension. Hairston faces an August court hearing and isn't enrolled in summer school. The team is in the offseason so Williams says he will wait "until the process is complete" to decide on the punishment.
Hairston, a 6-foot-5½ swingman, averaged about 15 points per game and decided in April to return for his junior season instead of entering the NBA draft. He was arrested on June 5 and charged with misdemeanor marijuana possession during a license checkpoint in Durham while driving a rented 2013 GMC Yukon.
According to rental records first reported by USA Today, the Yukon was one of two vehicles driven by Hairston and linked to Haydn Patrick "Fats" Thomas, a Durham man facing pending drug charges. Hairston was also cited for speeding in May while driving a 2012 Camaro rented under a woman's name using Thomas' home address. That vehicle was ticketed twice on the Chapel Hill campus for parking violations, once in April and once in May.
In all, four rental vehicles linked to Thomas or his home address have received a combined nine parking tickets on campus since February.
Last week, athletic director Bubba Cunningham issued a statement saying the school is still gathering information on potential improper conduct by members of the men's basketball team, though he didn't name any player specifically.
Williams had generally declined to comment on Hairston's case before Monday's statement.
"Our basketball program is based on great ideals, and these issues are embarrassing," Williams said. "These are not common in my 10 years as head coach at UNC and they will all be dealt with harshly and appropriately at the correct time to ensure that our program will not be compromised.
"We will care about each individual but there will be serious actions taken that will fix these issues. I take pride in our values and how we have conducted ourselves for a long time here at Carolina, and this time will pass but it will be dealt with strongly."
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