How do I stop Orca Screen Reader?

If you don't need assistance, you might want to consider:

sudo apt-get remove orca
sudo apt-get remove gnome-orca
killall orca

I say this from the unpleasant experience of accidentally triggering it on several occasions. It is a sledgehammer approach but I have absolutely no use for it.

If you want to keep it but it's starting on boot now, short of hunting through the application to look for the off-switch, consider this question too.

Also note that the package name is transitioning from gnome-orca to orca. I'm leaving both above for the duration of supported releases.

From the manual for Orca:

Insert+Q quit orca.

I've filed a bug about the poor user experience that results from Orca's current behavior.

Alt+Super+S also works to turn Orca off (or on).

This keyboard shortcut is configured as follows...

1. Open "System Settings"
2. Select "Keyboard"
3. Select "Shortcuts" tab
4. In the left panel, select "Universal Access"
5. In the right panel, select "Turn screen reader on or off"
6. Enter a new key combination to toggle Orca on or off.