You don’t. Welcome to owning a member of the vulpix family.
Edit: Once again, my editor has informed me that there has been a rule about one-line responses for quite some time, and this is a hard rule, not one I can violate whenever I merry well please as I do with other rules such as the laws of physics.
Similar to the zangoose ask a few posts ago, it’s a good idea to understand why your ninetales might be targeting specific types and how to remedy the particular reasoning behind it. However, unlike the zangoose ask a few posts ago, ninetales are known for doing terrible things to other people “just because,” which may be why there is an extremely popular legend in practically every known culture about how pulling a ninetales’s tail will result in a thousand-year curse.
That said, if there really is no reason behind your ninetales’s behavior besides sheer amusement and a certain level of malice against specific types, then most definitely add meditative exercises to your daily routine. However, unlike the zangoose ask, your aim with these meditative exercises is strictly to teach your ninetales how to heel for periods at a time—discipline, in other words. Once she figures out how to heel, you may take her out to socialize with other pokémon, particularly the grass- and bug-types she tends to burn. If she refrains from burning them, give her a treat to reinforce what she had learned from meditation. If she sets them on fire, add another ten minutes to your next meditation. Ninetales are exceptionally intelligent pokémon; eventually, she’ll begin to realize why you’re forcing her to sit still for periods of time and either learn to listen to you … or set you on fire instead. At the very least, if she does the latter, then she may tire herself out too much to set nearby grass- or bug-types on fire.
Alternatively, she may benefit a little more than the previous anonymous’s pokémon from being paired with the types she supposedly hates. By taking in a grass- or bug-type (or even both) and making them be part of your family, she’ll be forced to confront and perhaps befriend her new partners. Of course, it’s also important to be there during their first few interactions to ensure she doesn’t set them on fire as well, but eventually, after quite some time of this, she’ll break down and come to accept her teammates.
Best of luck, anonymous!
The Pokédex According to Bill 