Victor Lindesay, an IT programmer and lecturer at Reading University worked at CCGXM, a strategic consulting company, on companies' websites including the supermarket Safeway, holiday agent Saga and Cable and Wireless.
Using his old access codes after his contract had been terminated by CCGXM because they 'were not happy with his work', he entered clients' web sites and sent insulting e-mails to their customers, told investors that Saga had 'gone bust' and otherwise defaced the appearance of the sites.
Mr Lindesay was not paid for his last weeks' work or for the one week notice period stated in his terms of agreement.
He told Southwark Crown Court in London he had intended to cause "a couple of thousand pounds worth of inconvenience," however the court put this amount between £9,000 and £26,000.
Mr Lindesay admitted three charges of unauthorised modification of the contents of a computer.
The court heard how Mr Lindesay has already lost contracts because of his conviction worth about £20,000 and that there are certain contracts he cannot now undertake because he has a criminal conviction.
Passing judgement, Justice Christopher Elwen said: "This was an act of pure and unmitigated revenge for what you saw as a slight."
Mr Lindesay was sentenced to nine months in prison.
Richard Powell, Shout99