How do I fix overly yeasty kefir?
I can not give you an answer to your question, but whenever I had problems with kefir, I was told to thoroughly rinse the tuber and restart the kefir. 
On this German Website they recommend to restart with 200ml of milk and 1 tablespoon of lactose for a recovery of the kefir. I do not know if this could change the yeast to bacteria ratio.

On a sideline, this Russian website recommend a ratio of 107 CFU/g of lactic acid bacteria to 104 CFU/g of yeast for health reasons. If you don't get the yeast to bacteria ratio down, maybe a fresh start with a kefir with 104:107 could help?