Finns just don't ask. :) That's the beauty of it. Strangers anyway.

I might just have to stare at them blankly turn around and walk away then.

That is one good option. Or you could stare a while, shout: "Do not harass me!" and then run away. Same thing if someone asks: "How may I help you."

Some customer service people have tried to Europeanise themselves and have started to ask questions that have more than two words, but if somebody would ask me, I would strongly advice to stick to this kind, or very similar talk between a customer and a sales person:

– Ja sulle? (For you?)
– Tarttis kameraa. (Needs a camera.)
– Mimmone? (Whatlike?)
– Hyvä ja halpa. (Good and cheap.)
– Ei semmost ookkaa. (Ain't those.)
– No mitäoh? (Whatyogot?)
– Tämmöne. (Thislike.)
– Jaa. (Um.)
– Joo. (Yes.)
– Mitä maksaa? (How much?)
– Pari hunttii. (Couple hundred.)
– Onkse hyvä? (Any good?)
– Joo. (Yes.)
– No määotase. (I'll take it.)
– Tosso. (There.)
– Tattis. (Thanks.)