With this Swantide Workflow, require reps to enter in all the Key BANT Fields prior to converting a Lead. The Validation Rule works off another Workflow called "Formula Field to Check BANT Complete on Leads" which when deployed uses a Checkbox to determine if BANT has been completed.
So for example, if you already deployed the Workflow "Formula Field to Check BANT Complete on Leads" and your Variables Inputs were along the lines of what is in the below screenshot, then on the Error Message you would want to say something along the line of "Please enter Budget, if the Lead is the Decision Maker (yes or no), is there a Need (yes or no) and what's their Timeline is before Converting the Lead"
However, the error message can be anything you wish to display, just make sure you enter in all the required fields that make the Formula Field "BANT Complete" to equal true.
Once this Workflow is Deployed, the following Validation Rule will trigger if the BANT Complete Checkbox is False when Converting the Lead!