Ugh. I had to be this person at a department store where I worked. I nearly had a panic attack over the sheer rudeness of the approach, because every single cashier had to ask (and get three no's) anyway. There was also no way to get people properly signed up away from the register or customer service, so it was a pretty stupid thing to do anyway. I apologized to every person I approached for what corporate was forcing me to do, and silently wished that someone would chase whoever decided on this idiotic form of marketing around a store while they were short on time, shouting about credit cards. Worse still, my hours were decided based on how many people I got to sign up for the darn things.
There's not much you can do about the poor salespeople - probably they are also in the position that if they don't keep their credit numbers up, they won't get a good amount of hours, and won't be eligible for raises, bonuses, or anything. Write and complain to corporate, and get as many people as you can to write in as well. This constant credit pitching is intrusive, disruptive, and it's not a matter of dedication to get them sold - it's either good fortune to give someone a deal they can use, or forcing your employees to bully your customers into submission. Not great for customer loyalty.
