If you're following up and are not getting a response then set a limit to the number of times you'll try again. I personally do 3 follow-up email over the course of a 3-6 weeks. On the last email, I let them know it'll be the last time I follow-up and let them know how they can reach me if they'd like to talk in the future.

If you just keep following up and never stop, you'll likely annoy someone who is probably not interested in what you're selling and you'll likely get spam filtered.

Sometimes we just have to take the hint :)