The patron selfreg form should record the origin IP in the patron record
To inform the blocking of IP ranges from which bad actors are abusing the /selfreg form, record the origin IP in a varfield or new fixed field on the patron record.
If a varfield, the default of an x-Note would suffice, but make this varfield tag customizable via one of the administrative interfaces. Historically speaking, Admin Corner would have been the go-to place to make this customization.

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Andy Lechlak commented
We had to turn off our self reg form because of this issue.
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Shawn P. Farrell commented
This would be incredibly useful. So many bad actors these days, it's like watching an episode of Full House.