How to configure Views VBO action to disable relationships?
First part is easy. When you check the box 'modify entity values' in the VBO field, you should see an option to tick another box that says 'override label'. That lets you then set the button that users will see to be 'Disable relationships'.

The second bit - i think it would require more code for CiviCRM Entities module or civicrm_entity_actions. I say this because on a VBO i set up recently I had the option to tick both 'Add to Group' and 'Remove from Group' (and there was also 'modify entity values') - so it sounds like you need the options for 'Enable Relationship' and 'Disable Relationship' to be explicit options at that step.