prevent a package to be installed

[Gerard ROBIN <g.robin3@free.fr>]

Hello,
probably a stupid question: 
when I make an iso image I don't choose the package lightdm, but it
is in the iso image constructed. How can I prevent lightdm to be
installed ? (I use xfce4) 
I prefer to start in console and use startx if necessary.

(the version of live-build in jessie is 4.0.3-1 but I installed 
the 4.0.5-1)
 
tia

-- 
Gerard
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