Heat is radiated to space in form of infra-red radiation. Infra-red radiation is nothing else than long-wavelength light. And there is a theoretical concept for a spacecraft accelerated through a beam of light, called photon propulsion.
The great thing about photon propulsion is that it requires no propellant, that means that when you have access to an external energy source (like solar panels), you never run out of fuel.
Unfortunately it has a very, very low energy efficiency. This paper talks about just 20 µN of thrust per Watt, and that was for a directed laser beam under lab conditions, not infra-red radiation aimed away from the general direction where you want to go.
So yes, theoretically you can propel a spacecraft through directing its waste heat, but practically the effect is negligible.
There is also a larger-scale concept for a photonic rocket which is closer to your idea of using heat, the Nuclear Photonic Rocket. The idea is to use the heat generated by a nuclear reactor to generate thrust. But it's still not very efficient. A much better use of that much energy would be to use it to accelerate the reactor waste to generate thrust.