For me it is much better to use an external mouse than the touchpad, which I sometimes touch accidentally.
So, here is how to disable it with a short Perl-Script. A bash script with a bit of Perl would do the same, btw.
#!/usr/bin/perl
my $tp = `xinput list | egrep -i touch`;
chomp $tp;
$tp =~ s/.+id=(\d+).+/$1/;
system "xinput set-prop $tp 'Device Enabled' 0";
print "Touchpad disabled\n";
