To write off an investment, you will need to add an "Exit Transaction" (i.e. Stock Sale, Note Redemption) for $0 for each round you participated in. To quickly see which rounds of the company you're active in, go to the By Round tab found in Current Investments. Take note of the round names and the share amounts of stock, if applicable. If you use investment accounts, note which account has the holding. You may need to click on the round and open a transaction to see the detail.
Next, add a new transaction.
If you own stock, add a new Transaction for a "Stock Sale", where you sell all your shares of stock in the specified round at a share price of $0.00 per share (or whichever currency your portfolio is in). Be sure to enter your exact share count. If you purchased stock across multiple rounds, you will need to create separate stock sale transactions for each round of stock you own in the company.
If you have a note, add a new Transaction for a "Note Redemption" in the specified round, and include $0.00 as the Amount redeemed. If you have notes in multiple rounds, you will need to create separate note redemption transactions for each note round in which you participated.
Please note that in all "exit" transactions, the Exit Transaction box should be checked to indicate that your entire holding in the round has been exited. This box is pre-checked for you on the Transaction form.
Once exited properly, the company should appear in the Exits & Returns section of your portfolio.