forTo disable automatic time stretching when you adjust the tempo, you will need to change the "Tempo" parameter in the Track Inspector. Open the track Inspector by clikcing the "i" in the upper left of the arrangement view. You can also press the F4 key on your keyboard to open the Inspector.
While in the inspector, in the upper portion you will see the Tempo parameter on each track. You will want to change this option to be "Don't Follow." This now sets audio and midi events to not follow the global tempo and not timestretch.
For ease, you can select multiple tracks and choose this "Don't Follow" option and it will adjust for all selected tracks.
When you create a song for the first time, you do have the option of enabling/disabling automatic time stretch. In the new song dialog box, look towards the bottom and uncheck the "Automatically Adjust Tempo" option there.