There is a C clip underneath the crank. This diagram is from a Volvo, but it's the same for Miatas. You need to get two screwdrivers, or something thin, and push the clip off from the open side. Once you do that, the knob comes right off.
I pull the crank off as much as I can with some fabric underneath it then push the clip off with a flathead screw driver. Sometimes I push off the ends, other times I can get the screwdriver in the hoop part of the clip. Just take your time and be gentile.