Samuel's answer is very good, I just wanted to add another idea that can possibly make this work within a futuristic setting:
Radio wave cancellation: The idea of mimicking noise cancellation on the RF spectrum is good, but would be ineffective for all the reason's master_gibber described in his answer. By the time the canceller receives the communication and transmits the inverted signal, the original signal has already passed them by, and rendered their attempts at cancellation ineffective. In our current reality, there's no way around this...but in a futuristic setting, we have some other options.
A literal man in the middle: To cancel a signal being sent from Mars to Earth, an agent on mars could place a device on or near the transmitter (near enough to be able to communicate with it "instantly"). This device would be one half of a quantum-entangled "walkee talkee" that would communicate with an electronic warfare platform/ship at some point between Earth and Mars, to tell the canceller exactly what is being transmitted, and allow them time to prepare the inverse signal before the original signal reaches them.
Ideally, the canceller would be only far enough from the original transmitter that the radio signal would take exactly as long to reach the canceller as it would take the canceller to calculate the inverse signal. This way, the electronic warfare platform would be able to generate the largest cancellation shadow...though it would have no margin for error.
The man in the middle (or even the Mars agent) could then replace the signal with whatever message they wanted Earth to actually see.
Pros: This idea is based on a logical extension of technologies that already exist (though quantum entanglement is still in its infancy), so it wouldn't be too much of a narrative stretch to include this kind of application in a story set within the next century.
Cons: This raises the obvious question of "if there's quantum entangled communicators, why would you use radio?" The best answer (depending on the story you're planning on) is that one side of the conflict would have better technology/resources than the other. Alternatively, the nature of the signal being sent may require more than one recipient (since the quantum-entangled communicators would only be point-to-point).
An alternate idea: Depending on the nature of the signals being used, the same quantum entanglement device could be used without needing a transmitter in the middle. One agent could place the device near the Mars transmitter, and another agent could place the sister device on the Earth receiver, and cancel the signal after it's already been received, but before it's been demodulated.
This would still require a QE walkee-talkee, but the actual cancellation would require very little power, and is actually the sort of cancellation that some telecom companies currently do with the undesired portions of their own signals in-house.
If the signal was very directional, or could only be decrypted by that one receiver, then placing the device directly on the receiver itself would blind recipient, and allow an agent on Earth or Mars to inject data at will.