On your 6D (or any camera that does not have a built-in pop-up flash), use the menu, navigate to "External Speedlite Control" -> "Flash firing" -> "Disable"
See page 172 of your Canon 6D Manual.
Also note that the pattern works best on the center AF point. If a peripheral AF point isn't locking focus, switch to the center point. This will be particularly true when using wide angle lenses (being wide angle, the AF beam will seem to only appear in the center of the frame because the frame has a much wider angle of view than the spread of the AF beam.)
On a T3i (or any camera with a built-in pop-up flash) the menu for "External speedlite control" is named simply "Flash control". So you'll navigate to "Flash Control" -> "Flash Firing" -> "Disable".
See page 180 of your Canon T3i Manual.