Has a Celebrity ever cyberbullied/attacked a fan online (and if so, who)?

There have been a few documented incidents within which a celebrity has had a few harsh words to say to a fan on Twitter.  Attacked is probably a little too strong of a description, however, as the outbursts usually appear to stem from a reaction to negativity from a fan.  Here are a few examples of celebrity outbursts on Twitter.

Rihanna

Rihanna has been known to let people know what she thinks over Twitter in the past.  The most well known example of Rihanna attacking a fan came in 2012.

At the time, her fans were very critical of her relationship with Chris Brown due to the fact that he physically harmed her.  One unlucky fan let her know that "Making a song with the dude that beat your face off is not a good luck".

Rihanna was obviously not impressed with this, and hastily replied that "Neither is your avi", an obvious attack on the appearance of said fan. 

Lucas Leiva

Liverpool FC midfielder Lucas Leiva has also been known to react to criticism from the club's fans.  An unhappy supporter claimed in 2014 that "LFC can now announce the sale of Lucas Leiva for £3 and a bag of midget gems".

Lucas Leiva obviously took offence to this fan's tweet, and went out of his way to direct message him a message that read "With your mum you pr*ck". 

Lord Alan Sugar

Most famous for his hit TV show, The Apprentice, Alan Sugar is no stranger to a Twitter argument.  His most recent outburst occurred when a fan corrected the spelling of one of his tweets.  Alan Sugar's response was a heartfelt "Shut up grammar Nazi. Your type make me sick".  Not too friendly, I'm sure you'll agree.