Don't allow sending private messages to suspended users. Emails to suspended users should tell them how to respond, since they can't.

lib/email/message_builder.rb

@@ -1,4 +1,4 @@
-# Builds a Mail::Mesage we can use for sending. Optionally supports using a template
+# Builds a Mail::Message we can use for sending. Optionally supports using a template
 # for the body and subject
 module Email
 
@@ -26,12 +26,16 @@ module Email
                         user_preferences_url: "#{Discourse.base_url}/my/preferences" }.merge!(@opts)
 
       if @template_args[:url].present?
-        @template_args[:respond_instructions] =
-          if allow_reply_by_email?
-            I18n.t('user_notifications.reply_by_email', @template_args)
-          else
-            I18n.t('user_notifications.visit_link_to_respond', @template_args)
-          end
+        if @opts[:include_respond_instructions] == false
+          @template_args[:respond_instructions] = ''
+        else
+          @template_args[:respond_instructions] =
+            if allow_reply_by_email?
+              I18n.t('user_notifications.reply_by_email', @template_args)
+            else
+              I18n.t('user_notifications.visit_link_to_respond', @template_args)
+            end
+        end
       end
     end
 

lib/guardian.rb

@@ -215,7 +215,9 @@ class Guardian
     # PMs are enabled
     (SiteSetting.enable_private_messages ||
       @user.username == SiteSetting.site_contact_username ||
-      @user == Discourse.system_user)
+      @user == Discourse.system_user) &&
+    # Can't send PMs to suspended users
+    (is_staff? || target.is_a?(Group) || !target.suspended?)
   end
 
   private