Essentially, I had this debate for the light novel on the meat translator thing. That Marika is a brat and Aya is a brat tamer. I did this with my ex. The important thing in BDSM is to have a safe word, which they do mention in the light novel.
The line crossing really comes down to if the sub truly wanted to stop, they'd have to do it in the beginning before they may get into subspace. It doesn't seem that Aya has gotten Marika into subspace yet (where you essentially get high on the hormones from sex, like addicted to sex and become incoherent). But when Marika knew she wasn't stopping Aya, I mean there is a sort of play with brats where they are playfully disobedient and the brat tamer must "discipline" them, but it's like teasing. Just depends on how far they both want to take the teasing before "no" becomes truly "no". If anyone were to stop after they said "no" would the sub beg for more?
Haaaaahh. . . I miss being a brat tamer.