First of all, you state your bank is on “the” boycott list. Well, there is no one universal boycott list, nor is boycotting required in any way, shape, or form by the Catholic Church.
Formal or material cooperation in sin is something serious-- but any cooperation in the case of the bank giving money to offensive causes would be very, very remote. I don’t think it’s possible to boycott everything that is offensive unless you grow your own food, weave your own cloth, and get off the grid and homestead.
If you are going to boycott, you need to be specific, write to the company and tell them WHY you do not shop/bank/do business with them and what it would take to make you change your mind.
I presume you are referring to one of the boycott lists put out by pro-life groups. BOA is on several. So is Wells Fargo and most of the other major banks.
If you feel you have a viable alternative and want to boycott this particular bank-- then open an account elsewhere and write to BOA’s corporate office/CEO and Board of Directors (not the customer service group, which could care less why you close your account) telling them why you are closing your accounts.
I have boycotted businesses in the past, and still do when I feel called to do so. But when I do, I write and call to tell them I’m boycotting and why.