18 months is one of those ages where habits form more strongly. For paci use, by 18 mos a child no longer needs it for sucking comfort so it is a great age to be sure they are rid of it. Here's how we did it with 2. For the paci, we quickly made it naps/bedtime only, then bedtime only. We had a few discussions about how only babies need them, not big boys/girls..and big kids get to do (pick a couple of fun places,etc that can be a reward).
We then got rid of all but one pacifier and put a hole in the last one. We gave it to them, they immediately recognized there was something wrong, told us it was broken and we made them throw it away. That night went ok as we talked about the big activity planned for the next day. The following night when they asked for the paci, we reminded them it was broken and they threw it away. Never a problem again....and that was with both my son and my daughter.