You don't want kalk mix, you want kalk paste. After you mix the kalk if you OD it, then at the bottom of the container is this really thick white stuff, it's like a paste.
Turn off the pumps, powerheads, returns, anything that moves water. Take the white kalk paste (should look thicker then milk) in a syringe (I use one from an old test kit) and try to squirt the kalk in the mouth of the aiptasia before it closes. Then be relentless with it and cover the thing.
Leave the pumps off for twenty minutes or so, then just keep a return pump running for the next or or two. After that the thing will be dead.
I have done this many many times on many many tanks. Has always worked for me, my friends, my neighbors, there friends, well you get the picture. Part of me really thinks that products like Joe's Juice are just dehydrated kalk.
I have tried boiling water, lemon juice, white vinegar, Red Sea's products, and Joe's but the kalk has worked the best for me.
JME