hmm...I'd say if you try to "flush" it it might backfire sucking the weed killer ingredients into the area that your MJ roots are, but at the same time you would also be diluting the concentration of weed killer...an interesting question indeed...I guess one question would be which way do you think water would flow through your soil? And is it dry or wet soil at the moment?
I suppose if lets say your grow was on an incline and the area sprayed with weed killer was lower than your MJ plant, and your soil was dry, you could theoretically hook up a hose probably 3-4 feet "up" the hill from your plant and just turn it on and let it go...this might work if the soil is dry in the lower area so that the water would be pulled through your MJ plant soil into the dry lower area...
Fuck I don't know...*shrug*