But if you determine that targeted ads are worst than scattershot, the correct conclusion isn't to do scattershot, it's to invert your target group.
Unless you have a promo, you really don't want to advertise to them... But that is exactly what you end up doing, because you can't target "people that visited my website 6+ months ago".
When someone stops visiting your site, make sure to send them a email. Ask them to return! You have their email address, right?