The anger stems from pain of being misunderstood/misrepresented. Completely understandable, and one I know well. However, it becomes a force for real change when directed productively to address the issue it stems from. So, the best way to do that would be to do two things. The first one is to explain to them why their assumption is painful and how saying that is as painful as claiming stereotype againist Christians (such as being backwards idiots that have no value to the march of human prpgress) without bothering to get to know one. The second is to explain how curses are a way to give power back to those who have been stripped of it and greatly harmed (doesn’t matter if you truly believe that, we are using a form of manipulation here), much like some of their Psalms, and that the customers do not fit that criteria, therefore there is no reason for you to do so. You can sweeten it by saying that, if anything, you have blessed the pens for those who use them to have a good day, regardless of what God they believe in.
You will need to keep your voice and the atmosphere calm while you do so. In the past, I would buy the individual lunch or a drink so there was a reason to sit down and have a conversation about it.