Reputable vendors do test for bacteria and mycotoxins (toxins from mold or fungus). It is generally safe and you don't need to do anything to disinfect it. But if do you encounter kava that is moldy, damp, or smells extra funky, don't drink it. Adding disinfectant won't help: it might kill some of the bacteria, but won't remove the toxins produced by bacteria or mold. Report the situation to the vendor.
It's true that kava has some natural anti-fungal activity, but that does not mean that it can't get moldy. There have also been rare cases where toxic bacteria have been detected by govt inspectors in kava, so that can happen, just as with any other natural plant product. So if you are concerned at all, ask your vendor about what tests they do for microbials and mycotoxins, and be sure to purchase from vendors who do that regularly...