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Let me know.
The way we have solved it is to limit the number of transactions per hour from any single IP address. (We have typically set it to 3.) Spammers will change their IP address here and there, but this prevention seems to keep them at bay.
I have added this feature to the latest version of the add-on, which is NOT yet uploaded. I have the Concrete 5.6 version (2.1.1) ready, but not the Concrete 5.5 version (2.0.1). I'll post the 2.1.1 update right now, in case that fits the bill. (I'm not sure when I'll get to the 5.5 version.
The change is found in the settings area, where you can indicate how many transactions per hour you want to limit any given IP to.
Sorry for the difficulties. As mentioned - we are seeing this happen a lot as well. Hope this solution helps.