So, here's the trick and the truth. You can't simply eliminate bad behavior - its so impossibly difficult...but, you can replace it. So, instead of saying, "Stop!" or "Hush..." or "I'm sending you to Nambia" here's an idea or two...
1. As soon as she screams you could hurry to her side and try to make a different noise - loudly "Ooooooo" or right in front of her face match her scream with a "La,la,la,la,laaaaaa." Remember, we are wee ones mirrors - and the youngest ones simply need a new habit - and you can give it to them.
2. You could also try turning a scream into a silly noise by running your horizontal finger up and down between the lips...I have no idea what its called...the way you make it sound like you are talking under water - you know - though...the silly noise (oh, this is just going to drive me crazy thinking of a name for this action/sound).
3. Or stick your finger in her mouth and tickle her tongue - pretty annoying, even to a 18 month old. Soon enough she might associate something not so nice with an open, screaming mouth. And say something like, "Oh, you wanted me to stick my finger in your mouth." PS. If she likes it when you do this - don't go with this - it could quickly turn into a "fun" game - abort mission.
4. Final idea, yell back - of course - not shouting but mimicking. You know, the copy game, it might make it worse - but one try wouldn't hurt.
Painting: The Scream by Edvard Munch