Probably try to explain a bit about Scientology (major objections) and how you do not oppose their beliefs, but only their practices.
If they claim to be a Scientologist, try to ease your way out of the situation passively; don't be aggressive or violent or angry.
There have been a few times I've been approached on the street by people interested to know why I'm protesting and what I'm about and they turn out to be Scientologists. I don't try to "get out" of the conversation, I just calmly explain that they're welcome to be a Scientologist or anything else they want to be. I'm not protesting the individual, I'm protesting the organization. That there are some wonderful things that Scientology provides, but the good things they do and the good intentions of many of the individuals are overshadowed by the wrongdoings and ill will of the ones in charge.
After explaining myself in that manner, I've actually had more than one Scientologist thank me for explaining myself and take whatever flyer I happen to have.