Did you at one time have a talk with them and let them in? Or I wonder if a friend of yours gave the missionaries your name. Sounds like they have you on their follow-up list if it is happening that much. Full-time missionaries regularly knock on doors in their areas they are assigned, but maybe only once or twice a year. If it is happening more than that - esp multiple times per month - then there is a reason they are following up with you. 
 
I would just be very clear, but courteous, that you are not interested and don't want any more visits. They should respect that. 
 
Jett

If for some reason you were referred to the missionaries by a friend or family member it could explain there visits.  
 
They are not coming by on Saturday to insult your observance of the Sabbath, Saturday is a day when they try and catch up with most of there referrals if they are not able to contact them by phone, seeing that most people are home on Saturday and Sunday.  
 
I know they might of stopped by several times, they are not trying to offend you and if you would politely explain to them that you do not want to learn about Christ or his Gospel I know for a fact that they will stop coming by, at least if they are Missionaries of the Church of Jesus Christ of Later-day Saints. I have seen many times when people say Latter-day Saint missionaries are stopping by and it is members of other Churches.  
 
We do not try and visit people once they state that they have no desire to hear our message. I this is clearly stated to the missionaries this will stop the multiple visits per month.  
 
Now you may still get a visit a couple times a month because missionaries are rotated every 6 weeks, so about every 6 months you have a completely new set of missionaries in an area.