It is possible to 'jam' the receiver. But as you have told, it is nearly impossible to decrypt and translate the data or gain control of the USV.As it is almost impossible to crack the security codes of communication transmissions, NO! If it were possible, then there would be no Link-16, IFF, Peace Eagles, Predator Drones, Loyal Wingman etc etc.
And manufacturers are bound to introduce sensors and algorithms which the USVs will be deploying once communication is lost (thus all required to have autonomous sailing ability by processing AIS data and image processing). Or, as last resorts, actions to be taken platform once it decides that situation is irreversible. These are the stuff they refrain from sharing online.
Nice engine they have got there. This is the first step of true unmanned long patrol or 'loyal flotilla member' for big ships.