How do you turn the flash off on the canon 1000d?

There are two options.

Option 1: you turn the big twist-button, where your on/off switch is located, and turn it until it is on the No Flash mode.

Option 2: When you use one of the modes A-DEP, M, Av, Tv or P, you should click on the Menu button and in the fifth [of the seven] page[s] you find Flash Control.

When you've clicked on Flash Control you got to disable Flash Firing.

If this all doesn't help you, you have a little flash button right next to your lens.