Well, since you invited them I think you should at least be polite enough to welcome them nicely. However, if they start talking religion, I would just say, "I hope you didn't take my invitation to come over as an indication I am interested in Mormonism. Now, would you like some cookies?"
If they persist, then I'd politely say, "I really have no wish to discuss religion. If that is all you would like talk about, then we really have nothing left to speak about. Thanks for coming by." as you stand up and start walking to the door.
Believe me, if they are missionaries, they are used to the RUDEST behavior. They will not likely take offense personally if you are kind but firm.
Being kind but firm is the Christian way, I believe. If you are firm in your own faith, then it doesn't matter what they say to you - they are not going to sway you, and being rude them them is probably not what Jesus would do...just my opinion there...
When these folks stop by my house I always say hello to them on the porch, then kindly say something like "I really admire you for taking on your mission and believing so strongly in your own religion. However I am a member of XXX church already and and very comfortable with my beliefs. thanks for stopping by"
I do admire them - it takes a lot of guts to go up to perfect strangers day in and day out to talk about your religious beliefs. Many of us aren't comfortable speaking about our beliefs in our own churches or families, so I do think highly of their motivations. I just don't have any interest in hearing a sales pitch for another belief system...