These people are used to having a lot of customers availing their service, so their general course of action is to pick up the bags after they are filled. A simple "no", or even a winking of the eyes should convey to them that their help is not needed, and they generally move on to the next customer.

Obviously, they are human and appreciate the additional tips, so they do follow "familiar" faces occassionally. A "no thanks (with a smile!)" should take care of that ;-)