How to prevent unwanted purchases and avoid transaction fees on my e-commerce site?

[HelvieKnivesLLC]

I currently use Shopify's "Fraud Filter" to deal with "problem customers" more so than just fraudulent activities. Since I deal with specialty items, this app works well for me.  However, I'm charged processing/CC fees with every transaction, even though the order is immediately canceled and refunded.  I can understand this if the transaction occurs with Paypal for example, but these transactions are taking place in Shopify. Some of the same people I have "frauded" to keep from purchasing from my site, troll my website to try to purchase items and the fees mount up, costing me money! How can "these people" be kept from making the purchase in the first place?  I should not have to deal with, and pay, all of these fees when I don't want to do business with the people that I have "frauded" within this app.  I look forward to some feedback.