Here's what I've done successfully to remove aiptasia.
1) Remove the LR/coral that is infected.
2) Scrape off the aiptasia with a sharp, pointy tool.
3) Take a Q-tip, dip the tip in some lime juice (like the one that comes in those lime-shaped containers), and dab it on the area to kill off any remaining pieces (i.e., the foot).
4) Dunk the LR/coral in a bucket of tank water to remove any excess lime juice.
5) Retun the LR/coral to the tank.
Never had to kill one in the tank, so I don't have any recommendations for that.