Voiding documents is now easier than ever! Simply navigate to the document in question and press 'edit > void document':

After a document has been voided, it will still be accessible to your client via the original link and their Client Portal; however you will both now see the VOID stamp. Also, if the document was not completed, your contact will no longer be able to do so.
Important Notes:
Voided documents cannot be un-voided
An Invoice which has been voided may no longer be paid
A Contract which has been voided may no longer be signed
A Questionnaire which has been voided may no longer be answered/submitted
A Quote which has been voided may no longer be accepted
You may void an uncompleted, completed (paid/signed/accepted), or partially completed document