I left my old office before Christmas 2011. Former boss still gives me more than 10 emails a day and asks me to work without payment.

I am a foreign worker in US and had VISA, based on working with her. After working four and half years with her, I was very tired to work with her and decided to leave the office. If my old boss or colleagues had troubles for my old projects or works, I understand to help them. I am filing new VISA now and waiting for the process of this. My former boss realized that I could still work for her without salary, aka volunteer until having new VISA by new employer and kept annoying me all time after I left there.

Former boss cut my salary in the middle of Dec 2011 and I did not receive the full amount of salary which ruined my faith and respect to her... Even I worked over 90 hours per week in her office.

I had already emailed her that I was busy many times and politely mentioned I was not interested in working with her anymore.
Does anyone know these are legal? How can I stop her email bothering?

Its legal for her to keep asking... you are under no obligation to even answer her much less work for free.

Its not like she's making terroristic threats or anything which would be illegal..

You can simply add her email address to the spam filter of your email... and let it go straight to the junk folder.

I agree and you can just stop reading and replying via email.