What you shouldn't bother doing is repeatedly emailing them, or trying to get in touch in general. If they want a cake they'll order one. If they don't, you're wasting your time and probably being a pest.
This urge to bother people probably comes from the idea that you have to do every cake you're asked about. This is a false assumption.
What a lot of people fail to see is that if there's no deposit, it's not a customer, it's an inquiry. That person may have made a lot of inquiries. They might have decided not to get a cake at all. They're not your customer unless they've hired you.
This is actually an argument for collecting deposits, and payment in full before the cake is baked. I'm always mindboggled when I see yet another post on facebook about someone who's been stuck with a customized cake because the "client" never showed up to get it. When asked if they had been paid, the answer is generally no. Then why did you make the cake in the first place? I don't get it.
We were in the mattress store recently, and there was a guy in there arguing with the clerk about when he was planning on paying for his mattress. He kept insisting that he would pay when they delivered it, and the clerk kept telling him that they make each mattress to order, so they need to be paid ahead of time, He would listen to her explanation, then say "Okay, I'll pay for it when you deliver it." After about ten minutes of this the clerk was maintaining her composure, but I was ready to go smack the guy for her.
If you order something custom for you specifically, it is generally accepted that you'll pay for it ahead of time. If someone won't give you payment up front, that's not a customer, it's someone asking about a cake. Asking, not buying, Buying involves money passing from them to you.
Don't waste your time trying to call people who have asked about a cake. When you respond to their inquiry, give them a time limit to get back to you and all of the information about putting down a deposit. Then let them be a grownup who can make their own decisions. They'll call you if they want to hire you. And you can use your time for something more productive.
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com