Well, I had to put some plastic mesh over the venturi to keep little histy noses out. I could take it off now they're big. I don't think you need to do anything but silicone would work to plug them.
If you do not intend to utilize the venturi action you can plug the hole with silicon as Nancy suggests. A short length of plastic tubing that fits tightly on the oultet nozzle can cover the other hole without infringing on the water flow in the pipe itself and negating some of the "tuning". These modifications really work.
I think 1 of my powerheads came with a plug, I probably lost it now. My Rena XP3s adjustable flow jet head thingy also has a plug but I lost 1 there too. Shoots water straight up during water changes.
I've never used the venturi feature on PHs or filters, I'm not into bubbles but I'm not sure I really see the need? Doesn't seem to increase or reduce flow as far I I can see? Am I missing something good?