If it is indeed only the knob continuing to spin and not the whole shift lever , try to separate the mounting piece from the knob by pulling up on it while it's screwed onto the shaft. Then just solidly glue the two parts back together.
hmmm... ok. so you're thinking its the insert with the threads thats detached from the wood shift knob thats causing the spinning. Makes sense, I won't break it by doing this?
I really like my shift knob, don't wanna mess it up.