views/foos/_form.html.haml
= f.simple_fields_for :bars, @foo.bars.not_belonging_to_a_certain_baz(baz_id) do |p|
= render "bar_fields", f: p
Bar.rb
scope :not_belonging_to_a_certain_baz, ->{ |baz_id| where("bars.baz_id != ?", baz_id) }
Or if you are not able to use scope, you can check each bar separatly when render bar_fields partial:
views/foos/_form.html.haml
= f.simple_fields_for :bars, do |p|
= render "bar_fields", f: p if p.object.baz_id != baz_id