According to this site:
Stalking is a pattern of repeated and unwanted attention, harassment, contact, or any other course of conduct directed at a specific person that would cause a reasonable person to feel fear.
Stalking can include:
Repeated, unwanted, intrusive, and frightening communications from the perpetrator by phone, mail, and/or email.
Repeatedly leaving or sending victim unwanted items, presents, or flowers.
Following or laying in wait for the victim at places such as home, school, work, or recreation place.
Making direct or indirect threats to harm the victim, the victim’s children, relatives, friends, or pets.
Damaging or threatening to damage the victim’s property.
Harassing victim through the internet.
Posting information or spreading rumors about the victim on the internet, in a public place, or by word of mouth.
The bit about “directed at a specific person” seems to be crucial in distinguishing stalking from certain kinds of advertising.