1. Open transitions editor
All added files appear on the main layer. To turn off the transition effect, click the Transitions option.
To disable transitions for one file only open the editor with the button and use the Transitions option also.
2. Turn off transitions
Click the "No transitions" button in the transitions editor.
These are all steps required to disable transitions, now you can click the "Make Video" button. :)
Instead of the "No transitions" button, you can set the duration of the fade in and fade out transition effects separately with the "Duration" parameter. The value "0" completely disables the transition.