You do what you would on a terrestrial reactor, and shut down the reactor e.g. by using control rods, or pulling out fuel rods to shut down the chain reaction. These wouldn't be simple radiothermal devices, but proper actively managed reactor cores.
Having said that you absolutely would need active cooling even in a 'shut down' state, probably by circulating a cooling fluid to radiator fins. You might use the same fluid as the propellant or might not. In the former case sure, you wouldn't be able to use all of it as propellant.
Having said that you absolutely would need active cooling even in a 'shut down' state, probably by circulating a cooling fluid to radiator fins. You might use the same fluid as the propellant or might not. In the former case sure, you wouldn't be able to use all of it as propellant.