If it's a standard E-2 visa, you can't work until it expires anyway whether you cancel it or not, as I'm assuming that your former employer will not grant you a letter of release.
If you plan on returning to Korea, your best bet is to wait out the remainder of the original visa period and return on a new one.
As T_Rex mentioned, your former employer has no legal footing to sue you for anything (unless you left bills unpaid that she had to cover etc).
Also, I'm not sure what the legality of transferring a contract over to another school is. Chances are, if you didn't sign a new contract etc, then it wasn't official and you aren't bound to it.
Block her, wait it out, and come back for a fresh start.
