I see you know nothing about cars.
There aren't "brake cables" under the car aside from those that articulate the parking brake. Cutting a parking brake cable would do nothing to the braking ability of the automobile in a situation like yours. Brakes are hydraulic these days and, thus, in order to cripple someone's braking system you would have to kink and/or destroy the metal lines that are nested (usually) high in a painfully inaccessable part of a vehicle's frame or floor pan.
Just thought I'd educate some of the lesser proficient car people.
-Mike-