I've done this by creating 2 calculated fields and a parameter.
Parameter - to drive the user selection
A calculated field to identify the city you don't want - this will return true if city is C otherwise false
Another calculated field to use on your filters shelf to control the behaviour. This will return True if the parameter is true OR the city to exclude field is false (ie the city isn't 'C')
Drop the above on the filters shelf and set to True. Changing the parameter between true & false will then give you what you need.
PS - I have moved this post to the main Forums (Forums ) area of the Community which is where it will attract more attention. The area of the site you've posted in is generally for questions about how to use the Community site itself. Please be aware of this when you post your next question.