If you no longer have access to the Facebook page, but still can see it in your Manychat account, there is a very quick solution to this problem. Disabling the bot for this page will not help since you don't have sufficient rights to reload this account. What you need to do now is to leave this page. First, click anywhere outside of the "Reconnect Page" window:
After that head to Manychat Settings → General, scroll all the way down, and click the "LEAVE" button. This way you will delete yourself from this bot, and it will be removed from your Manychat account. If you are an owner of the account, the ownership has to be transferred to another admin before leaving the account.
You can also remove the current Facebook page that is connected to your Manychat account and connect another Facebook page. Head to Manychat Settings → Messenger. Keep in mind, that all your Facebook subscribers will be deleted from Manychat.
