How do you teach an 18-month-old child not to hit?

Well, this answer is to keep those who disapprove spankings happy (although a spanking wouldn't really help in this situation as you are doing exactly what you are trying to teach your child not to do).

They recommend 1 minute time out for each year of life. So an 18 month old would get a 1.5 minute time out. I would recommend using a playpen, something that your child couldn't escape from. You wouldn't want to use the crib because the crib is associated with sleeping and you don't want your child to associate time out (punishment) with the crib. For every time your child hits, you tell him/her "no, do not hit" and place them in time out regardless of screaming and crying. "Ignore" the child while he/she is in time out. Supposedly, after doing this so many times, the child will begin to associate hitting with time out (punishment) and hopefully quit doing so. Whatever you choose to use as the time out space, make sure you use the same spot every single time at home. Pick a similar spot for when you are visiting others (a small playpen can easily be packed in the trunk). Whichever form of discipline you use, also make sure that you never slip. Keep it up, no negotiations, every single time the child hits, even if he/she attempts to hit and misses.