Taxonomy term reference field only from top level terms of a vocabulary
If you replace your taxonomy term reference field with an Entity reference field you will be able to use the Reference field option limit module to restrict the available options for dependant fields. If you need to apply further restrictions you can also generate your options lists via special entity reference views.

Note however that by switching the field type to entity reference you will no longer be able to use taxonomy term specific fields and filters for those fields in views.