I learned a technique to turn away high-pressure salesmen years ago, that works like a charm. This same technique works well with JW's. Know first off what you want. "I am not interested in your product." Expect that several objections and questions will be posed to you. Always answer the same, regardless of what is posed to you. After about five tries, the most persistent salesman will give up. A typical conversation goes something like this:
Seller: Good evening ma'am, are you interested in saving money? (instruction sheet, get the householder to say "yes")
Me: What are you selling?
Seller: Just this week we have a special offer in your neighbourhood that can save you hundreds and thousands of dollars. Do you like saving money, ma'am? (instruction sheet, get the householder to say "yes". If she says "yes" three times in a row, she might say yes to the fourth.)
Me: What are you selling? If you don't tell me now, I am hanging up. (Daughter in background. Do you have to be so rude, mom?)
Seller: (Thrown off sales pitch, run finger to pitch ten) Um, uh, for this brief time only I am authorized to offer you this deal that saves you thousands of dollars in real estate fees if you buy a time share in Musky Hollows Cross Country Ski Resort, located in a year-round paradise of Ice and Snow, the untouched wilderness of Antarctica.
Me: I am not interested.
Seller: But ma'am, did I mention that you receive, absolutely free, a pair of genuine sealskin mukluks..
Me: Mukluks are tempting, but I am still not interested.
Seller: Ma'am, I am only one buyer short of my quota. If you buy this time share, I will receive a bonus which will help me finish my graduate degree in cosmetology. You don't want to disappoint an energetic young buck like myself, do you, so close to a promising career? Stuck in this dead-end job the rest of my life?
Me: That is too bad, I feel for you, I do, but I am still not interested.
...and so on.
Like others have said, you do not owe strangers an explanation for your decision, it is completely your own affair.
Scully, that illustration is priceless, and so true! I have seen that triumphant smile more than once in my local Kingdom hall. Double points if they get the person to baptism ("Let's prepare for the District Assembly!")