It means "Don't. Now means now" or "Don't. Enough means enough", so I guess you can figure out by the context. You say it when you are angry with someone that isn't doing what you want them to do. I'm picturing a mother angry with her children and telling them to stop or shut up but they keep complaining. It's used in these kinds of situations.

Just like this guy from Venezuela has just explained to you, this means that doing something AT THAT MOMENT you are talking IS A MUST, I mean...it can not be postponed. For example: madre: lava tus manos, ya. --- Mother: wash your hands, right now. hija: lo haré en cinco minutos, somanete quiero comer un ratito. --- Daughter: I am going to do it in five minutes, I just want to eat for a bit. madre: No, ya es ya... vete y lava tus manos. De lo contrario, te vas a enfermar. - Mother: No, no way.... go and wash your hands. Otherwise, you are going to get sick. I hope I helped you, if there is anything else I can help you, feel free to ask me. Have a nice day.

YA ES YA is an expression we use a lot in Venezuela and it means that you have to do it right away, inmediately, no excuses. Example: madre: tiende tu cama ya hijo: en un momento lo hago madre: no. ya es ya, ve y tiende tu cama ahora mismo

I partially agree with Estefanía: it would mean something like "You must do it now, no more excuses". However, it doesn't mean exactly the same as "Enough is enough", where we would say something like "¡Ya está bien!" or "¡Para ya!" (in Spain, at least). A bit of context would help, though.

Good question