I've had a few in the past few years. I've always used the same method as I kill Aiptasia. I mix Pickling Lime with RO water to a toothpaste consistency, aka Kalk Paste. I use a plastic medicine syringe and inject this paste into their mouths and coat them with it. This seems to kill them fast.
Before killing it, see your other thread. There's a chance we're all wrong and it's not a Majano. I think it is...but have been known to be wrong before....many times...lol. Wouldn't wanna kill something that's a great addition to your reef.
I've got a really cool looking green one that I tried to kill with boiling water from a syringe and it popped out of another hole in the rock! I had two and the first one shrivled away when we zapped it with boiling water so I suppose that method has a 50/50 chance. I'll try and take a picture of the green one that survived...for now.