It's sad isn't it that as full grown adults who are volunteering their time to do something (like give a talk) cannot say they don't want to do it anymore without having to make up a load of stories and excuses as to why they don't. We've all been there, some have the courage to just walk away, for others it's so much harder. This is what it is to be in a cult... you have no freedom or choice like normal people do.
Tell yourself you have the right to choose what you do and don't do in your life and that you don't owe anyone a reason or explanation. It's very freeing!