The bots automatically clean up their knowledge database nightly if they go over the limit. They will delete garbage data and old conversations.
If you go over the limit, the bot will still function, but importing will be disabled. If you go way over the limit, the bot will be disabled until it can clean itself up.
You can also run the clean up task "forgetfullness" from the bot's Knowledge page to force a clean up.
You can also "pin" knowledge or responses to ensure they are never deleted.