Hi Pretty!
I noticed that since Im older, I am more up front and straight up about the way I feel now. I am not rude or mean, but I believe in making eye contact and getting to the bottom line. I suggest either of the two approaches.
"Hi Ms. Stylist (Insert correct name),
Im here for a wash and blow dry set--whatever whatever. I "Do Not" want a trim at all."
(If she tries to suggest a trim anyway, you can use rebuttals such as you actually just had a trim or you use practice healthy hair care at home and trim on an "as needed" bases.)
OR
"I have a serious serious scissor phobia. I don't like for people to come near me with scissors. I warned my last stylist, but she didn't listen. I was looking in the mirror and saw her come near my head with scissors. I
started trembling and
kicking and screaming and punching! Turns out-- she thought I was playing. "
I would go with suggestion one, but the other one is alright too.
HTH