Non-staff users may not delete their posts in archived topics.

lib/guardian/post_guardian.rb

@@ -77,6 +77,9 @@ module PostGuardain
     # Can't delete after post_edit_time_limit minutes have passed
     return false if !is_staff? && post.edit_time_limit_expired?
 
+    # Can't delete posts in archived topics unless you are staff
+    return false if !is_staff? && post.topic.archived?
+
     # You can delete your own posts
     return !post.user_deleted? if is_my_own?(post)
 
@@ -107,4 +110,4 @@ module PostGuardain
   def can_vote?(post, opts={})
     post_can_act?(post,:vote, opts)
   end
-end
+end

spec/components/guardian_spec.rb

@@ -833,6 +833,22 @@ describe Guardian do
           Guardian.new(coding_horror).can_delete?(old_post).should eq(false)
         end
       end
+
+      context 'the topic is archived' do
+        before do
+          post.topic.archived = true
+        end
+
+        it "allows a staff member to delete it" do
+          Guardian.new(moderator).can_delete?(post).should be_true
+        end
+
+        it "doesn't allow a regular user to delete it" do
+          Guardian.new(post.user).can_delete?(post).should be_false
+        end
+
+      end
+
     end
 
     context 'a Category' do