Is it possible to exclude only sulfur?
It is ok to skill with sulfur but dont get more sulfur ore.

You'll have to change line 524-526 to this:

		void OnDispenserGather(ResourceDispenser dispenser, BaseEntity entity, Item item)
        {
            if (!initialized || !enableDispenserGather || entity == null || !(entity is BasePlayer) || item == null || dispenser == null || item.info.shortname == "sulfur.ore")​

The only thing I added is 

It would be pretty trivial for the plugin owner to make this a config option, but not many people must ask about it. 

You can also use GatherManager to fix your sulfur gathering rate. You don't then have to worry about modifying code every time there's a ZLevels plugin update. It's what we've done on our 10x servers.

Like this:-

  "Options": {
    "GatherDispenserModifiers": {
      "*": 10.0
    },
    "GatherResourceModifiers": {
      "*": 10.0,
      "Sulfur Ore": 5.0
    },
    "MiningQuarryResourceTickRate": 1.0,
    "PickupResourceModifiers": {
      "*": 10.0,
      "Sulfur Ore": 5.0
    },
    "QuarryResourceModifiers": {
      "*": 10.0,
      "Sulfur Ore": 5.0
    },
    "SurveyResourceModifiers": {
      "Sulfur Ore": 5.0
    }
  },​