This has happened many times. I usually tell them thank you but I have already tried it, and they either take it well or seem somewhat offended. The ones that know me know that I really have tried it and I know what I like and dont like- the new ones will often reach for another that I have already tried and then give up after about 4-5 tries. Some get all uppity that you're shooting them down but the majority are pretty good about it.
If you have the time, a good response is to ask them what they think of it and just chat with them.
Funny story, Lush have pretty aggressive SA's and they follow you around and start up a sales pitch over anything you so much look at. Anyhow, this one girl just wouldn't let up and kept going on and on about this bloody perfume. She was going on about how much she loved it, the sale price and how they only had 2 left. I saw my opening and went in for the buzzkill...
"You really seem to love it, so much more than me, I dont think I could take the last bottles from you. I'll leave it for you and at that price you really would be mad not to get it!"
She really didn't know where to go after that, and kind of frittered away out of sight.