When and by whom was the word "neoliberalism" or "neoliberal" first
used with today's negative connotation and why was this particular
label chosen?

This doesn't answer the question, but it's a start:
http://web.inter.nl.net/users/Paul.Treanor/neoliberalism.html

This is a very challenging question. 
Here are a few more bit and pieces which I hope will help:

"First used to describe Chile’s successful market reforms after the
1991 "Washington Consensus," neoliberalism became a mantra for market
reforms that were to unleash growth. But something odd happened on the
way to the millennium. Neoliberalism became, for many Latin Americans,
a synonym for cronyism, insider privatization, and especially the
corruption that marked the Carlos Menem (1989-1999) and Alberto
Fujimori (1990-2000) regimes in Argentina and Peru."
http://www.miami.edu/nsc/publications/newsupdates/Update52.html

"Andrew Norton, of the Centre for Independent Studies in Sydney, has
observed that the term emerged on the political left in the 1990s"
http://www.nzbr.org.nz/documents/speeches/speeches-2002/neoliberalism.doc.htm

NEOLIBERAL NEWSPEAK
http://sociology.berkeley.edu/faculty/wacquant/neoliberal.pdf

" . . the term 'neoliberal' means many things to many people.
Moreover, it is a loaded and often pejorative word. It is for that
reason that I am striking it from my vocabulary and urge others to do
the same."

Summary of comments on use of the term "neoliberal"
http://csf.colorado.edu/forums/ipe/99/msg01100.html

Leli