To create an invoice or receipt with 100% VAT, go to Sales > New Invoice or Purchases > New Receipt. Add a line to the invoice:

• Quantity – 1.
• VAT Rate - anything other than "n/a".
• VAT Amount - the value of the VAT.

The VAT rate you select in the drop-down box is only used to automatically calculate the VAT amount for you so by following the above example you're just overwriting the automatically calculated VAT amount.