diff --git a/gencapdefs.py b/gencapdefs.py
index 31045839..10c9481b 100644
--- a/gencapdefs.py
+++ b/gencapdefs.py
@@ -171,6 +171,12 @@ CAPDEFS = [
         url="https://wiki.znc.in/Playback",
         standard="ZNC vendor",
     ),
+    CapDef(
+        identifier="Nope",
+        name="oragono.io/nope",
+        url="https://oragono.io/nope",
+        standard="Oragono vendor",
+    ),
 ]
 
 def validate_defs():
diff --git a/irc/caps/defs.go b/irc/caps/defs.go
index 2d086227..e4fcfcac 100644
--- a/irc/caps/defs.go
+++ b/irc/caps/defs.go
@@ -7,7 +7,7 @@ package caps
 
 const (
 	// number of recognized capabilities:
-	numCapabs = 26
+	numCapabs = 27
 	// length of the uint64 array that represents the bitset:
 	bitsetLen = 1
 )
@@ -116,6 +116,10 @@ const (
 	// ZNCPlayback is the ZNC vendor capability named "znc.in/playback":
 	// https://wiki.znc.in/Playback
 	ZNCPlayback Capability = iota
+
+	// Nope is the Oragono vendor capability named "oragono.io/nope":
+	// https://oragono.io/nope
+	Nope Capability = iota
 )
 
 // `capabilityNames[capab]` is the string name of the capability `capab`
@@ -147,5 +151,6 @@ var (
 		"znc.in/self-message",
 		"draft/event-playback",
 		"znc.in/playback",
+		"oragono.io/nope",
 	}
 )
diff --git a/irc/handlers.go b/irc/handlers.go
index 801cd8c6..dbd964f1 100644
--- a/irc/handlers.go
+++ b/irc/handlers.go
@@ -574,6 +574,13 @@ func capHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *Respo
 			}
 		}
 
+		// #511: oragono.io/nope is a fake cap to trap bad clients who blindly request
+		// every offered capability:
+		if toAdd.Has(caps.Nope) {
+			client.Quit(client.t("Requesting the oragono.io/nope CAP is forbidden"), rb.session)
+			return true
+		}
+
 		// update maxlenrest, just in case they altered the maxline cap
 		rb.session.SetMaxlenRest()