I see the problem presented by the OP, especially since the person buying isnt following but keeps going by the last helper. I had one a while ago as well but he has departed willingly (I guess he just got helped by others :P), no special tactics were used.
As for followers etc, mine are mostly in the low 15-30 ish levels and while some might find that annoying I adore these *starters*. They apparently love my constant selling of low level stuff (I do make a lot of breads/popc etc :P) and I sell loads of animal produce at semi discount prices. My idea on those items is just depleting silo/barn and some profit+xp and who buys those I simply couldnt care less. In return for me often helping them with boats like 40 corn, 11 milks etc or whatever else they need (and I dont care about loosing lol :P), they often throw those same low level items in their shop with the added bonus of them not using as much sugars/dairies. Basically I can buy their overflow sugars/dairy and make them some cash for buying up other items/crops. Its a win-win, I get to restock items I want, they get the money they need.
EDIT:
Btw when a randomer from DD helps me in a particular helpfull fashion (for instance a long to produce cake or jam), I also like to buy to buy out their shop of *undesireables* like chilli popcorn etc. Would that constitute me as a stalker as well :P?


.



