The venturi adjustment would be on the airline hooked up to your pump which mixes the bubbles with the flow.
As a general rule it's better to draw darker, dense skimmate rather than wet IMO
Closing off the knob slightly will make the foam climb higher and dryer. If this halts skimmate production instead of drying it out (darker skimmate) you may need to increase air and/or decrease bubble size as well. If that also does not work, then the problem may be design related (neck too wide for action relative to the air and water volume running through the unit).
What pump is powering the skimmer?