Currently, if you set up a sync between Smartsheet and Resource Management, you can only detach the integration from the Smartsheet side. I have an instance whereby the Smartsheet project has been deleted and can no longer be restored. In Resource Management, the corresponding project still shows a link to Smartsheet (if you click the linked Sheet link in RM, it will tell you the Smartsheet is no longer available). However, I can no longer re-link the Resource project to another Smartsheet as there is no mechanism to break the link. I can no longer access or restore the Smartsheet to break the link.
It would be nice to be able to remove the link from the Resource Management UI.