--- a/hgext/acl.py	Wed May 30 12:55:33 2012 -0700
+++ b/hgext/acl.py	Mon May 28 19:45:15 2012 -0300
@@ -174,7 +174,21 @@
         return True
 
     for ug in usersorgroups.replace(',', ' ').split():
-        if user == ug or ug.startswith('@') and user in _getusers(ui, ug[1:]):
+
+        if ug.startswith('!'):
+            # Test for excluded user or group. Format:
+            # if ug is a user  name: !username
+            # if ug is a group name: !@groupname
+            ug = ug[1:]
+            if not ug.startswith('@') and user != ug \
+                or ug.startswith('@') and user not in _getusers(ui, ug[1:]):
+                return True
+
+        # Test for user or group. Format:
+        # if ug is a user  name: username
+        # if ug is a group name: @groupname
+        elif user == ug \
+             or ug.startswith('@') and user in _getusers(ui, ug[1:]):
             return True
 
     return False