Unfortunately, you can be absolutely banned from or refused employment at a company you used to work at. Private employers are under no obligation to hire anyone; they can decide for themselves whom to hire, and can refuse to hire someone for any reason that is not specifically made illegal. The illegal reasons to not hire, moreover, are very few and specific; for example, no discrimination against people in employment on account of race, age over 40, sex, disability, or religion; no retaliation against someone for using a legally guaranteed right, like FMLA leave, or filing a legally protected claim, like a wage and hour complaint. Outside of those few specific categories, however, employers are free to decide who they will hire and who they will not, and for what reason(s).