I had similar problems with Jehovas Witnesses some years ago. They always turned up around lunchtime on Sundays. I used to see them gathering outside and being given their assigments. They HAVE to take part in the doorstepping as part of their commitment. If you want to enter a discussion, question the fact that they are being forced to do the doorstepping. If they have children with them, make a point of asking the child what they had for their birthday, as they don't allow the celebration of birthdays. They will soon go away.
There was a young woman who died this week after refusing a Blood transfusion after childbirth, as she had been forced to believe a twisted interpretation of a bit of the Bible.