I wonder how much this is going on within the chain. If the witlesses start interfering too much with their business, they should be kicked out of the property (unless, that is, they are there to shop and not to preach). Most stores have a rule against soliciting and distribution of literature. If they are in fact violating this, you might get results by complaining to the store manager or the police. If they are excessively pushy and will not let you through unless you waste your time on them, informing the store manager should get interesting results.
And, if that doesn't work, one approach that will work if enough people do the above is to follow up a second complaint with a threat to boycott the place if the witlesses are pushy a third time, and prepare to follow through with your boycott. If they know they are going to lose customers because of the witlesses, they might start banning the witlesses--especially after they abuse the privilege enough times and get too many complaints about being excessively pushy. You can shop at Target, at a local store that will not tolerate the witlesses, or online. Walmart has little that I can't find better online: Vitamins, electronics, decorations, Christmas lights, toys, and clothing are usually available elsewhere and in better quality. L.L.Bean has better clothing, outdoor equipment, and bed and bath supplies than Walmart. Stonewall Kitchen has better cookware than Walmart. Christmas Lights, Etc. has better lights. 1000 Bulbs Inc also has better lights (and not just holiday lights--they have lights for everything). You can find better hardware, houseware, and electronics online as well as toys.
Just beware that you are going to spend some more money. If you take this approach, expect to pay 2-3 times what you would at Walmart. However, the payback is that it will last you more than 2-3 times as long, reducing your costs in the long run. You will thus be hurting them for allowing the witlesses to push shoppers around far longer than they might expect, since it will be that much longer before you need to replace products and because you might decide to spend more money next time and get longer lasting, better quality items in the first place.