One of my customers needed to disqualify about 700 dead leads that had been imported from their previous CRM solution.  Unfortunately, there isn't any automatic way to do this, but after Googling around a bit, I found a fairly good solution that doesn't require any coding.  Follow these steps:

As the CRM Administrator:

1) From the CRM Server, launch the Workflow Manager.

2) Create a Manual Rule on the Leads Entity called Disqualify Lead that will contain the following actions:

3) Save and Activate the rule.

As a CRM User:

1) Perform an Advanced Find on the Leads Entity to identify the Leads you wish to Disqualify.

2) When the results display, select those records you wish to Disqualify.

3) From the Leads Grid toolbar, select More Actions, Apply Rule….

4) From the Workflow Process dropdown list, select the Disqualify Lead rule you created earlier.

5) Select OK and CRM will change the status of the Lead from Open to Disqualified.

6) Open the Closed Leads view to check your work.

Note: This process ( steps 3-6 ) will work with any list of Open Leads that you may be viewing, not just after performing the Advanced Find operation.