You need to use a datapack for that mod, and set the replace value to true so that it wipes the tag.

I tried adding items to a tag and it also did not work. I even tried enabling the kubejs script.js example still with no change in game.

btw, there's no real issue, the tags i tried to remove were from BCLIB and removing tags or recipes from this mod is just impossible with kubejs

Example code used:

onEvent('tags.items', event => {
	event.remove('c:iron_ingots', 'betternether:cincinnasite_ingot');
  	event.remove('c:shears', 'betternether:cincinnasite_shears');
})

and if I understand it correctly #c:iron_ingots and #c:shears does not exist at the moment where the tag event happens and the tag event creates a TagWrapper with an empty proxylist.