validates_uniqueness_of question

Try the "if" option, mentioned here:
http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#M000816

So you could, for example, add one of

:if => "!canceled"
:if => Proc.new {|invitee| !invitiee.canceled }
:if => not_canceled? # where not_canceled is a method in your Invitee class