While there is no bulk way of reverting QBO back to pre-Avalara Connector installation, you can do the following to unattach Avatax from QBO.
1. Go to Apps > My Apps > Avalara, and press Disconnect.
2. In the invoices where there has been tax calculated by Avalara, you can click the trash can icon to the right of the line item which added tax, and delete the tax line.
3. In the "Select a Sales Tax Rate" window below the subtotal, change the type to something other than Avatax.
The transaction should be back to it's original form.