To delete ALL of a bot's knowledge and responses go to its Admin Console, Knowledge, "Status" page.
Click on "Delete All", this will delete EVERYTHING in your bot including ALL responses, scripts, and knowledge.
If you do not wish the bot to learn new responses, you can disable learning from its Learning & Settings page. Learning is recommend to be disabled.
It is also possible to delete all a bot's "unpinned" data. Learned data is not pinned, so this would delete learned data. Note, it may also delete responses and other data if you did not pin them. Responses entered through the Training & Chat Logs page are normally pinned.
To delete all unpinned data you can edit your bot's details a decrease its "Knowledge limit" to a low number such as 10,000. You can then go to your bot's Knowledge "Reports & Tasks" page and click on "Run forgetfulness". This will delete all unpinned data. Be very careful doing this, as it may delete your bot's responses, important data, or corrupt your bot. You can then reset your bot's knowledge limit.
You can also export the responses you want your bot to have, delete all or create a new bot, and import the response list back.