Thanks for your interest in PixelCNC! Modifying the post-processor is very simple. Make a copy of your metric Marlin post processor to edit so you don't permanently modify your existing post - unless you're confident you'll never need the original.
Open it up and at the bottom are the definitions for various command block types. You will see the set of commands for SpindleOff/CW/CCW followed by their definitions in double-quotes. Simply remove all the text between the double quotes for each of the three 'Spindle' block definitions, leaving only the starting/ending double-quote characters. This will effectively render any and all spindle command blocks null and void so that whenever PixelCNC internally sees that it's when it should emit a command for the spindle it will just output nothing instead. Easy peasy!
I would suggest also removing the text between the double-quotes for the ToolChangeBlock, so that it too is just a pair of double-quotes. That should remove any tool-change command blocks from outputted G-code as well.
Let me know if you have any further questions and keep your eye out for v1.34a on August 1st, big things are happening!
Good luck!