any copper is bad. I still think the CO2 is your best bet because it can only help the plants but it will kill the cherry shrimp. $20 for a paintball tank, $3 for CO2 refill, and you don't need a regulator or special equipment if you are going to just "gas" the shrimp. Find a cheap paintball CO2 connection and empty the entire CO2 tank over a 24hour period? whatever maximum rate you can find without blowing your substrate or soil out of the water. Your plants will be fine, especially when you do the water change, no lights (prevent algae), and I would think based on the dead shrimp carcasses on the bottom of the tank you can judge whether this did the trick or not.
Any chemical warefare makes me wary, so I say if you going to invest money into higher grade shrimp and don't want the cross breeding, spend a little more for the CO2 "gassing shrimp" set up (which will be less if you are not using the system for CO2 plant injection).
Just my 2 cents.