diff --git a/src/dataclassish/_src/flags.py b/src/dataclassish/_src/flags.py
index b3f04ec..acc3fa2 100644
--- a/src/dataclassish/_src/flags.py
+++ b/src/dataclassish/_src/flags.py
@@ -2,7 +2,7 @@
 
 __all__ = ["FlagConstructionError", "AbstractFlag"]
 
-from typing import Any
+from typing import Any, final
 
 
 class FlagConstructionError(Exception):
@@ -36,5 +36,6 @@ def __new__(cls, *_: Any, **__: Any) -> None:  # type: ignore[misc]
         raise FlagConstructionError(cls.__name__)
 
 
+@final
 class NoFlag(AbstractFlag):
     """No flag."""