You can't regulate the CO2 produced by yeast with any kind of clamp/valve/screw - you'd just cause an extremely sticky explosion.
Once the yeast in your reactor is producing CO2 there are only two practical ways you can regulate it:
Temperature: All life processes (including yeast's production of CO2) get faster as the temperature increases (within the tolerable range for that organism). Most processes double with an increase of 10 degrees C.
Some people (including me) utilise the heat generated during the day by the light's starter units to increase production during the day, and let it tail off a bit at night (ie, I just stand the CO2 bottle on the starter unit - low tech!)
Other people (especially in cold parts of the world) keep their yeast generating unit (the bottle, etc) in a bucket or tank of water containing an aquarium heater - they can then regulate the production of CO2 by changing the temperature the heater is set to.
Apart from temperature, you can regulate the amount of CO2 dissolving in the water by changing the efficiency of your diffusion methods. To reduce CO2 levels you could raise a glass diffuser higher in the tank, place it in an area of minimal flow, or with Hagen's diffusion ladder you can insert the CO2 tube higher up the ladder (there are three places to put it.
I suspect from the fact that you're asking about clamps you're looking to reduce the amount of CO2 in the tank water, so in your case raising the temperature of the reaction chamber isn't going to help, and you'll have to look at reducing the efficiency of the diffusion method.
Are you sure you want less CO2? is it causing the animals a problem? If not, I'd just let keep going as it is.
With more information on what you want to do people might be able to give you better help.
Best,
Mark
Edit: As London Dragon says above, you could also run an airstone at night if you want to reduce the levels of CO2 in the tank at night. I was just looking at ways to regulate the CO2 going into the tank, but if lower night levels of CO2 are what you're after then London Dragon's suggestion is the easiest one to automate.