I hate, hate, hate HATE being tickled. My skin is super sensitive and when someone tickled me it hurts, it hurts a lot. When I was a child I used to get yelled at by my parents for screaming in pain when some jerk would think it was funny to tickle me. I was being "over dramatic". No, I was screaming for help because some jerk was hurting me. I don't tickle my friend's kids, I have too many horrible memories of tickling to ever inflict that torture on a child.
Now I'm an adult and I make the rules about my body. people who tickle me get one, and only one, very firm and very serious warning about they are never, EVER to do that again. Period. If they think it's funny to try and tickle me after I had my preferences clear, they get what they get and I will ask them "Now, will you respect my boundaries?" (thankfully very few people feel the need to push the no tickle rule since the people I am friends with are adults who respect boundaries".
Don't be gentle with your initial warning. You don't have to yell or curse, but don't smile or laugh it off either. Look people straight in the eyes and say firmly "Do not ever tickle me again, this is your warning". I find people back off from tickling after that.