Hos to stop my printing machine when I started for too many pages

There is several way to do it. I suppose that you have only one computer...
Ubuntu was installed with "cups".

Cups manage and share printers on your computer with other computers on your network. So, Cups is listening on the port 631. You can verify on your computer. In a terminal, enter:
netstat -tapn | grep 631

You will have something like:
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN -

So you can use your browser to connect with http://localhost:631
And you can manage your printer on this interface:
- change parameters
- Stop tasks assigned to your printer (this is what you ask)

In fact, i think you can do the same thing with the printer manager installed with Gnome, but i never used it...
Perhaps someone will give you another answer...

Cornet Odile,

an other way to do it is:

<Applications> >Accesoires >Print_Manager

double-click on your printer,

click on the task you want to stop,
and choose stop-task...

It takes a small while before the printer stops, because there is a bit in the printer-buffer.

I am still unable to cancel printing despite clicking on to Applications and then Accessories. Print Manager has not appeared.