Just use the report button. They're breaking Etsy's TOU, and if every seller did this just about every business would be harmed, and people would be less likely to buy on here since we'd all seem pushy. They're not just harming themselves, they're harming us, too.
If I get messages like this, I tend to send them a message back to warn them that it's spam, but I report them anyway. Etsy is less likely to take action against them if they stop doing it right away, even if they're notified, but if they continue to do so something will happen. And even if I don't report them, others probably will instead.