You could try to edit
Code:
/usr/share/X11/xorg.conf.d/50-synaptics.conf
Can't remember right now the option to change but it's quite guessable if I recall correctly.
Edit :
First, you might want to work on a copy of the file
Code:
cp /usr/share/X11/xorg.conf.d/50-synaptics.conf /etc/X11/xorg.conf.d/50-synaptics.conf
Then the option could be palm detection wich prevent the use of the touchpad while typing (try increasing the value of palm dimensions first ?)
Edit 2 : synclient to have a look at actual config ?