在Android中，可以使用BluetoothAdapter类的cancelDiscovery()方法来取消蓝牙设备的发现，并使用BluetoothSocket类的close()方法来断开已连接的蓝牙设备。

以下是一个简单的示例代码，展示如何断开已连接的蓝牙设备：

import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothSocket;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;

import java.io.IOException;
import java.util.UUID;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "BluetoothDisconnect";

    private BluetoothAdapter bluetoothAdapter;
    private BluetoothDevice connectedDevice;
    private BluetoothSocket bluetoothSocket;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 初始化蓝牙适配器
        bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();

        // 注册广播接收器以接收蓝牙连接状态的改变
        IntentFilter filter = new IntentFilter(BluetoothDevice.ACTION_ACL_DISCONNECTED);
        registerReceiver(bluetoothReceiver, filter);
    }

    // 连接蓝牙设备
    private void connectDevice(BluetoothDevice device) {
        connectedDevice = device;

        try {
            // 获取设备的UUID
            UUID uuid = device.getUuids()[0].getUuid();

            // 创建蓝牙套接字
            bluetoothSocket = device.createRfcommSocketToServiceRecord(uuid);

            // 连接蓝牙设备
            bluetoothSocket.connect();
        } catch (IOException e) {
            Log.e(TAG, "无法连接到蓝牙设备", e);
        }
    }

    // 断开蓝牙设备连接
    private void disconnectDevice() {
        try {
            // 关闭蓝牙套接字
            bluetoothSocket.close();
        } catch (IOException e) {
            Log.e(TAG, "无法关闭蓝牙套接字", e);
        }
    }

    // 广播接收器，用于接收蓝牙连接状态的改变
    private BroadcastReceiver bluetoothReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();

            // 检查是否断开了蓝牙连接
            if (action.equals(BluetoothDevice.ACTION_ACL_DISCONNECTED)) {
                BluetoothDevice disconnectedDevice = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);

                // 检查是否是已连接的蓝牙设备
                if (connectedDevice != null && disconnectedDevice.getAddress().equals(connectedDevice.getAddress())) {
                    Log.d(TAG, "已断开蓝牙设备连接");
                    connectedDevice = null;
                    bluetoothSocket = null;
                }
            }
        }
    };

    @Override
    protected void onDestroy() {
        super.onDestroy();

        // 取消蓝牙发现
        if (bluetoothAdapter.isDiscovering()) {
            bluetoothAdapter.cancelDiscovery();
        }

        // 断开蓝牙设备连接
        if (connectedDevice != null) {
            disconnectDevice();
        }

        // 注销广播接收器
        unregisterReceiver(bluetoothReceiver);
    }
}

上述代码中，connectDevice()方法用于连接蓝牙设备，disconnectDevice()方法用于断开蓝牙设备连接。在Activity的onDestroy()方法中，取消蓝牙发现并断开蓝牙设备的连接。还定义了一个广播接收器来接收蓝牙连接状态的改变，并在断开连接时进行相应的处理。