If you are using the UI, you may not know where the RouteRule section is. See the blue arrow below -- click on that "Default" text to see the entire proxy file.
You'll probably see a RouteRule section already there that includes the TargetEndpoint but no condition. I have added a RouteRule before it with a name of "NoTarget" and a Condition but no TargetEndpoint. The RouteRules are evaluated in order from top to bottom, with the first matching condition being chosen. If the condition is satisfied, that target will be used. With no TargetEndpoint, there will be no target called. If there is no condition on a RouteRule, it always matches.
So, for the example above, if the request is GET /quotastats
, there will be no target called. Otherwise, it falls through to the second RouteRule, and the default target will be called.