If a topic has run its course (or if it has grown contentious and nothing further can be gained by keeping it going), but you want to keep it available in the message archive, you can prevent further replies from being posted to that topic by locking it. If desired, when you lock the topic, you can send a Locked Topic notice to inform members that the topic is now locked. 

Members who send email replies to a locked topic receive an automatic delivery failure notice stating that the topic is locked. 

You can lock or unlock topics through their properties. For more information, see Editing the properties of topics in the archive.

Other ways to lock topics are:

• Use hashtags that have locking properties.
• When you are creating a new topic or replying to an existing topic on the group website, select the Lock Topic After Posting checkbox before you click or tap the Send button. (This checkbox appears only for group owners and moderators who have the Edit Archives permission.)

Tip: Groups.io has a setting that allows groups to automatically lock topics that are older than a specified number of days. You might consider using this setting to prevent members from posting replies to outdated topics. See Automatically Lock Topics Older Than [n] Days.