I've had that happen a few times, and it was a turn off for me as well. I feel like if you want to offer a promotion for people that visit your shop, you can post a coupon code on your shop or item listings.
I've had people message me to give me props on my photography though and offer me kind comments, and I'll usually offer them a coupon code for making my day, but I never message someone first offering them a coupon code since I know that's against the Etsy rules.
Having said that, I wouldn't go as far as reporting someone for sending me a coupon code, that seems kind of harsh and mean if they're just trying to make some sales. If it bother's you that much, try writing to them instead offering a different way to promote.