Stop is NOT a good indicator, nearly everyone says stop when they are being tickled, even if they like it. If someone knows the person likes being tickle "tortured" and is doing tickle "torture" for that person, then I don't think hearing a lone "stop" would lead them to believe they really need to stop. If its not the first time you were so miserable, then your obviously not experienced enough playing with each other - or others, and obviously need some set word or signal to clarify.
"Ouch" and "your hurting me", or - "stop it hurts" - would be good indicators that it is hurting however.
You see, with what you first post "yelps of pain" to me that doesn't say "oooowwwwwwww" or "ouch" I read that as general shrieks and screams just like people often do when they are tickled.