Many boxes that are for low voltage wiring are really nothing more than frames for the hole in your drywall. There isn't anyway to clamp the cable if you wanted to. It's good to support the cable with staples or ties but you don't have to clamp it to the box.
You should secure your cables so the LV and HV don't run right next to each other prior to entering the box.
You should secure your cables so the LV and HV don't run right next to each other prior to entering the box.