title: Delete a row from a dataframe if its column values are found in another
body: <pre><code>df1 = {
      'vouchers': [100, 200, 300, 400],
      'units': [11, 12, 12, 13],
      'some_other_data': ['a', 'b', 'c', 'd'],
      }
  df2 = {
      'vouchers': [500, 200, 600, 300],
      'units': [11, 12, 12, 13],
      'some_other_data': ['b', 'd', 'c', 'a'],
      }
  </code></pre>
  
  <p>Given the two dataframes like above, I want to do the following: if voucher from <code>df1</code> can be found in <code>df2</code>, and their corresponding unit is the same, then delete the entire voucher row from <code>df1</code>.</p>
  
  <p>So in this case the desired output would be:</p>
  
  <pre><code>df1 = {
      'vouchers': [100, 300, 400],
      'units': [11, 12, 13],
      'some_other_data': ['a', 'c', 'd'],
      }
  </code></pre>
  
  <p>What would be the best way to achieve this?</p>