You're talking about
this car pack.
Well, the siren lights of its police cars are made in a particular way (called ELS lights system), and that's done on purpose to get the new nicely looking siren lights effects that they have. So, it's normal for them that the police lights turn on when you use brakes.
The only way to "switch them off" is to edit the respective textures of the siren lights in their .ar files making them totally transparent, so to look invisible in-game. Anyway, do this will delete the particularity of the pack release, and won't give back the default siren lights glow balls, since they're not properly implemented in them (it's like if they're not used, but the objects are present just to give the police car property to them, that's the possibility to switch the sirens on and off).