what does "pin him down and sit on him"mean? what does "sitter"mean?
Kemp Clark was still trying. He and Rose had exchanged bitter words, and afterward Clark had taken the worried Price aside and advised him that he favored using force. "It may come to pinning him down and sitting on him," he warned, adding that he would be delighted to be among the sitters.

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Answer

To pin him down would mean to physically subdue him by force. To tackle him to the ground.
To sit on him would mean exactly what it sounds like, to sit down on top of him so he cannot move.