What very strange behaviour!!
The closest example of such vandalism which I have previously experienced was where a poster on another board started systematically deleting all of her posts (thus rendering much of the discussion incomprehensible) before departing that particular board. Unsurprisingly that vandal was another creationist.
I think the suggestion of Percy alerting Karls ISP of this behaviour is warranted, it might preclude a future board becoming the victim of such vandalism.