The periphrasis dejar de has several uses in Spanish. The first use shows the end of an action or habit.

Dejé de fumar el año pasado.
I stopped smoking last year.

He dejado de asistir a clase por falta de tiempo.
I stopped going to class for lack of time / because I didn’t have the time.

It’s also used in the imperative form:

¡Niño, deja de jugar con esa pelota!
Boy, stop playing with that ball!

¡Por favor, deja de molestarme!
Please, stop bothering me!

When used in the negative form, it shows:

– A repeated habit

No he dejado de venir un solo día.
I haven’t stopped coming one day. (I came here every single day.)

No he dejado de practicar un día.
I haven’t stopped practicing one day. (I practiced every single day.)

– An ongoing action

No deja de repetir lo mismo.
He keeps repeating the same thing. (on, and on and on)

No deja de hablar de ella.
He keeps talking about her.

– As a piece of advice or reminder

No dejes de llamarme.
Make sure to call me. /Call me.

No dejes de estudiar español.
Don’t stop studying Spanish. / Keep studying Spanish.