in the physics behaviour there is an action to turn of collisions..
From the manual:
These actions affect Physics behaviors in general, not just the one it was set for.
Enable/disable collisions
By default, all Physics objects collide with each other. You can disable collisions between the object and another Physics object so they pass through each other. This affects all instances of both object types. Note: enabling collisions again when objects are overlapping can cause instability in the simulation.