In cults like jehova's witnesses and other forms of christianity where they send out "missionaries" in the street or door to door, the idea isn't really to convert people but to strengthen the bond of the people who carry out the work and the rest of their tribe.
I can imagine there's an incredibly low success rate of speaking to strangers asking them to follow a religion, yet it's far more likely for a missionary to hear "F*** off you c*nt!" They start to realise the world is a hostile place towards their cult, they go back to their friends and say "everyone was mean to me when I spoke about our cult" and their friends comfort them, say things like "you're safe here with us" etc. It spreads this belief no one likes them but other cult members and they should strengthen their ties to said cult.
The best thing you can do is be nice but of course firm. I used to live round the corner from a Church of Latter Day Saints, got several knocks on my door and stopped in the street. Every time I answered "thank you but this is not for me, I wish you all the best though" and walked on/shut the door. I think it's more jarring to them to be let down nicely than aggressively!