St Polycarp, whose feast we celebrate today, gives us an excellent example of how to address heretics. The exchange is presented in the third lesson of matins for today's feast.
It is recorded for us by St Jerome, who writes:
Cumque ei fortuito obviam fuisset Marcion et diceret: Cognoscis nos? respondit: Cognosco primogenitum diaboli.
(When Marcion met Polycarp one day by chance and asked him 'Do you know me?', he answered, 'Yes, I know the firstborn of the devil')
(One way of dealing with heretics!)