I just tell them that I am opposed to Planned Parenthood and cannot support their effort and wish them the best of luck. For the same reason,I stopped supporting Race for the Cure (against breast cancer - which is also much more common among maternal abortion victims). I don’t tell the GS that though.
If you had to you could say: Thank you for offering, how nice. Unfortunately, I am pro-life. Take care. as one is walking away.
I wouldn’t do it, but that would be an option.