From the Record:
  1. In Records, Vendors, open the appropriate Vendor
  2. On the Vendor tab, change the status dropdown to Inactive
  3. Save and close the record

Globally:

  1. In Administration > Globally Change Records > highlight Vendors and click New Change
  2. On the General tab > select Status
  3. Use an operator of "Replace" and select Active > With > Inactive
  4. On the filters tab, filter on the Vendors you'd like to change by selecting them or utilizing a Vendor query
  5. Preview Changes
  6. If everything looks good > Change Now

Note: Marking a vendor as Inactive prevents adding invoices and creating payments for that vendor. To allow payments but not invoices, use the On Hold status instead.