A zoomatic strainer knob is pressure-fitted on at the factory. I found out today the tolerance between the shaft and the hole is 4/1000 of an inch. In other words, almost the same diameter as the hole! i believe the only way to get it off is pound it off, and then of course you run the risk of breaking more metal parts.
Occassionally you see zoomies for sale with the knob missing. It is compression fit and if you do get it off, you may want to put a drop of crazy glue on the shaft before you re-install. Some more info on Zoomatics: http://vintagedrumguide.com/zoomatic_strainer_repair.html