Militant that I am, I would go to the bank and ask them: "on what basis are you refusing these bills?" I have had a couple of run ins with banks now (actually self-important tellers who think they own the money supply). If they want to argue about the legality of marking bills, I have found that the 'in your face' attitude works best. "If you think I have broken the law, you need to call the police, if you don't, you need to apologize" works for me. I had one teller start spouting off nonsense about 'banks being authorized to protect Bank of Canada currency' I had to ask her what the Bank of Canada would think about a chartered bank refusing legal tender on this basis (a few marks on the bill). You could see her lapse into a confused state mutter something about 'we'll see about this', and started counting my deposit. I have found these folks will never be happy unless they think they can intimidate you. I'm too old to take that kind of crap anymore.