diff --git a/mobile/src/main/java/be/mygod/vpnhotspot/client/ClientsFragment.kt b/mobile/src/main/java/be/mygod/vpnhotspot/client/ClientsFragment.kt
index 3e341ded..5b159d59 100644
--- a/mobile/src/main/java/be/mygod/vpnhotspot/client/ClientsFragment.kt
+++ b/mobile/src/main/java/be/mygod/vpnhotspot/client/ClientsFragment.kt
@@ -32,6 +32,7 @@ import be.mygod.vpnhotspot.net.monitor.TrafficRecorder
 import be.mygod.vpnhotspot.room.*
 import be.mygod.vpnhotspot.util.ServiceForegroundConnector
 import be.mygod.vpnhotspot.util.toPluralInt
+import be.mygod.vpnhotspot.widget.SmartSnackbar
 import java.text.NumberFormat
 
 class ClientsFragment : Fragment(), ServiceConnection {
@@ -118,10 +119,14 @@ class ClientsFragment : Fragment(), ServiceConnection {
                 }
                 R.id.block, R.id.unblock -> {
                     val client = binding.client ?: return false
+                    val wasWorking = TrafficRecorder.isWorking(client.mac.macToLong())
                     client.record.apply {
                         AppDatabase.instance.clientRecordDao.update(ClientRecord(mac, nickname, !blocked))
                     }
                     IpNeighbourMonitor.instance?.flush()
+                    if (!wasWorking && item.itemId == R.id.block) {
+                        SmartSnackbar.make(R.string.clients_popup_block_service_inactive).show()
+                    }
                     true
                 }
                 R.id.stats -> {
diff --git a/mobile/src/main/java/be/mygod/vpnhotspot/net/monitor/TrafficRecorder.kt b/mobile/src/main/java/be/mygod/vpnhotspot/net/monitor/TrafficRecorder.kt
index f047abef..74fde249 100644
--- a/mobile/src/main/java/be/mygod/vpnhotspot/net/monitor/TrafficRecorder.kt
+++ b/mobile/src/main/java/be/mygod/vpnhotspot/net/monitor/TrafficRecorder.kt
@@ -146,4 +146,9 @@ object TrafficRecorder {
         unscheduleUpdateLocked()
         records.clear()
     }
+
+    /**
+     * Possibly inefficient. Don't call this too often.
+     */
+    fun isWorking(mac: Long) = records.values.any { it.mac == mac }
 }
diff --git a/mobile/src/main/res/values-zh-rCN/strings.xml b/mobile/src/main/res/values-zh-rCN/strings.xml
index dcfa6ed2..8f095bb3 100644
--- a/mobile/src/main/res/values-zh-rCN/strings.xml
+++ b/mobile/src/main/res/values-zh-rCN/strings.xml
@@ -59,6 +59,7 @@
 
     <string name="clients_popup_nickname">昵称…</string>
     <string name="clients_popup_block">拉黑</string>
+    <string name="clients_popup_block_service_inactive">拉黑需要为该接口打开服务。</string>
     <string name="clients_popup_unblock">洗白</string>
     <string name="clients_popup_stats">流量…</string>
     <string name="clients_nickname_title">%s 的昵称</string>
diff --git a/mobile/src/main/res/values/strings.xml b/mobile/src/main/res/values/strings.xml
index 1e2fc30f..575f3868 100644
--- a/mobile/src/main/res/values/strings.xml
+++ b/mobile/src/main/res/values/strings.xml
@@ -63,6 +63,7 @@
 
     <string name="clients_popup_nickname">Nickname…</string>
     <string name="clients_popup_block">Block</string>
+    <string name="clients_popup_block_service_inactive">Turn on service for this interface to block the client.</string>
     <string name="clients_popup_unblock">Unblock</string>
     <string name="clients_popup_stats">Stats…</string>
     <string name="clients_nickname_title">Nickname for %s</string>