Hi @azure,
Mattermost doesn't currently support this but do any of these feature requests sound like what you're looking for?
- Add a way to prune/reset/clear channels
- Automatically delete the oldest attachments when approaching a set storage limit
- Add actual permanent deletion - on database AND filesystem level
If so we'd love your help creating a UserVoice account and voting for the features that you would like to see implemented most. Feel free to submit new feature ideas as well if you don't think any of the existing ones address your issue. The feature requests on UserVoice have a large influence on the features we were work on for subsequent releases.