set $block_pc 0;
if ($http_user_agent ~* (Googlebot|Bingbot|Baiduspider|YandexBot|Sogou|360Spider|DuckDuckBot)) {
    set $block_pc 0; # 蜘蛛放行
    break;
}
if ($http_user_agent ~* (Windows|Macintosh|X11)) {
    set $block_pc 1; 
}

if ($block_pc = 1) {
    return 404; # PC端返回404
}

很简单的根据UA判定的

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