You could remove the weed by physical means, you could change environments to make them less attractive for the weed (add insects, more or less water, close off your harvest etc), you could genetically modify your monoculture, you could spray with substances, either synthetic or found elsewhere in nature, you modify the space between plants in your monoculture (by more or less space or by changes to soil), you can add various form of bacteria, or even nanoparticles, you could buy a goat (or robot goat, to be more modern) etc