Add breaks to prevent users from upgrading.

debian/changelog

@@ -1,8 +1,13 @@
-muffin (2.6.1-2~) UNRELEASED; urgency=medium
+muffin (2.6.1-2~) UNRELEASED; urgency=critical
 
+  [ Maximiliano Curia ]
   * Add rm_conffile rule for /etc/sgml/muffin-common.cat. (Closes:
     #787695)
 
+  [ Margarita Manterola ]
+  * Add Breaks for cinnamon, as this package is not backwards compatible.
+    (Closes: #788203)
+
  -- Maximiliano Curia <maxy@debian.org>  Thu, 04 Jun 2015 09:34:37 +0200
 
 muffin (2.6.1-1) unstable; urgency=medium

debian/control

@@ -64,6 +64,7 @@ Description: lightweight window and compositing manager
 Package: libmuffin0
 Section: libs
 Architecture: any
+Breaks: cinnamon (<= 2.6.0~)
 Pre-Depends: ${misc:Pre-Depends}
 Depends: muffin-common (= ${source:Version}), ${misc:Depends}, ${shlibs:Depends}
 Description: lightweight window and compositing manager (shared library)