When my maid's contract is about to end, she asked me for a reference letter to find a job. I tell her I will only give phone reference and keep the caller's phone number for future use. If she does a good job till the end, I will issue a nice ref letter and will not call her future employer.....implying if she does not behave, I will tell the future employer and her new job is be gone.
I will not do that, since u have to pay money in lieu of shorter notice (if less than 1 month). She will be even happier getting 1 extra month of pay without working.
Unless there is a criminal offence (stealing, beating, working in another job), Immigration will not stop her from coming again.