If you are wanting to disconnect Xero either temporarily or permantently there is an easy way to do this.

Log in to Xero

Go to Settings > Add-ons

Click on disconnect for Jobsheet.

https://go.xero.com/Settings/Addons/

Go back to Jobsheet and click 'update Xero' on the jobs list. This will fail, and the button should disappear.

The button to send invoices to Xero should have gone way on the invoice page as well.