Yes we can make her stateless and yes we have.
Can we prosecute her ? I'm not so sure we can. We cannot use evidence from phone taps so what is the evidence ? We can prove she left this country as someone can stand up in court and say so. After that ? Would we get evidence that can be tested in court that she left Turkey and entered Syria, more to the point, can we prove any illegal activity once she arrived there ? Evidence requires a witness that the defence can cross examine. I doubt we have that hence the removal of citizenship.