Is it possible to prevent two specific lessons from being taught simultaneously?

You can prevent two lessons from being taught simultaneously simply by adding the some resource - teacher or classroom, that is required by both lessons. If for example Math and Physics are taught by the same teacher, or they must be in the same classroom then this will happen automatically and software can not place such lessons on the same period.

Please see:
Adding and Editing a lesson
How to assign lessons to classrooms?

However, if you situation is differently - e.g. those lessons are taught by different teachers and they are placed in different classrooms - and you still do now wish them to be taught at the same time, you can use this approach:
Two teachers cannot teach at the same time