I, too, am a people pleaser by nature and find it hard to turn down orders. However, I've recently been able to say no to requests for larger orders that were initially discussed with me less than a week away from the due date (and I would need to ship). It was uncomfortable for me to say no to the order but I sure did rest easier that night knowing that I wouldn't be scrambling big time to fill an order. Plus, being under a lot of stress isn't the environment I thrive on to produce work that I'm comfortable selling.
Being asked to design something of an uncomfortable nature as in your situation would cause me to pause as well. I think it's important to make those decisions based on how you want to represent yourself and your business to others. That's just me...I think you've been given some great advice here 
. Best of luck to you!!!