Just add straight water to the reservoir instead of nutrient solution.
But flushing before harvest is over rated bro science.
It want hurt anything and will save ya some coin in nutrients but that’s about it.
But, there’s no scientific evidence showing any benefits. Most people like the idea of following closely to nature and nature doesn’t flush. Infact, any time actually blind taste tests have been done. The results suggest people prefer the unflushed weed… same goes for produce really. Fruits and veggies grew as closey to as they would be in their natural environment just taste better 
Google “blind taste tests of flushed and unflushed cannabis” if you’re interested… it’s pretty cool stuff to me 
I grow with bottom feeding and feed till harvest.
My current grow, mostly all autopots.