All you need to but on the letter to revoke the wage assignments is this "I herby revoke any and all wage assignments I may have signed with XX Company." That's all you have to put regarding that!
As far as them calling you at work, well, that's another story. These places are notorious for harrassing people at work. They will tell your co workers that you are about to be arressted for theft, and all sorts of other wonderful things! It's hard to get them to stop calling . . . I had the receptionist at my work tell anyone that called for me that I didn't work there anymore. Still, one pdl called 20 times a day for almost 2 months before they stopped, each time being told I didn't work there anymore.
One option is to tell your coworkers that your identity has been stolen by someone that took out a bunch of payday loans. That way you won't be totally embarrassed by the calls. I know it's lying, but it's better then losing your job or having all your coworkers gossiping about how you are going to be arrested.