Really, you don't have any recourse, but you probably won't have to pay the credit card bill, either.
First, I don't know what agreement you signed with Bank of America. It is possible that you agreed to pay if your employer did not pay. In that case, you will face some more difficulty.
Second, I am assuming that the purchases on the creidt card are business related, not for your own personal benefit. If you purchased items for your own personal benefit (not for travel expenses for your employer or the like), then eventually you will probably have to pay these charges yourself.
Third, there is no way for your employer to stop Bank of America from trying to collect from you, except to actually PAY the bill themselves. BoA has a right to TRY to get payment from anyone they think might be responsible. There is nothing that your employer can say or do to change this except to either (a) PAY the charges or (b) perhaps admit in writing that you are not responsible.
It is possible that you might be able to "tease" out of your past employer a statement -- in writing -- that they are responsible for the charges. This could be very helpful in the future if you get sued.
That is, if you keep writing to them, they might write something back without realizing that they are admitting somethign improtant.
However, you will probably just have to wait until you get sued, and then defend against that.
If you are forced to pay any money, you can turn around and sue your employer to recover this money.
However, you are running the D A N G E R of the 3-year statute of limitations running out to sue your employer.
Therefore, you probably should file a Warrant in Debt in General District Court suing your past employer for the money to pay and retire these charges on the BoA credit card.
NOTE: It is always possible that ther eis a geniune DISPUTE wtih BoA and your employer is trying to make BoA see that some of the charges are WRONG and should not be piad.
But you should be able to get to the bottom of this if you file a Warrant in Debt and sue your past employer.
You might want to WARN THEM that you "hate to do this" but the statute of limitations may run out so your hands are tie.d
If you get sued, you would again file a "Warrant in Debt" or "thirde party motion for judgment" suing your ex-employer.
That is, you would say in court whatever BoA collects from me, my employer should pay instead."
NOTE: As far as the harassment goes, if a debt collector were doing this, they would be violating the Fair Debt Collections Practices Act.
However, BoA can collect for itself without coming under the FDCPA.