There is nothing that can be done about this.
Generally, I don't send offers to people who are watching my items - it hardly ever results in a sale. Often, watchers are not interested in buying the item, they may have a similar item to sell or may just forget to delete their watch list from time to time.
Sometimes I get watchers on items I have for sale for under £5 - I am never going to send an offer for such a low priced item. However, watchers are not worth getting stressed about. Just ignore them.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)