Since the plastic spinners - in addition to the difficulties you are experiencing - are prone to breakage fairly easily and when they go the pieces go everywhere, I'd suggest going with one of the previously suggested aluminum spinner nuts, like the DuBro ones. To add to the safety issue mentioned, if you are having to crank down on the nut as hard and as tight as I get from your description, you may have already introduced cracks - even small enough that you can't see them - that will be promoting the previously mentioned destruction.

When dealing with spinners that are spinning around 12,000 RPM, I take safety over looks - and besides that, I like the look of the DuBro spinner nuts!